Critterhunter
New member
I had this discussion elsewhere, but figured it would be of use here as well in the broader sense of detectors in general and the pros/cons of using rechargeables compared to non-rechargeables, so I'm re-posting a compilation of replies here together on the various topics of battery use, care, and chargers with detectors...
[size=x-large]First, the disclaimer- Use any advice at your own risk to person, property, or so on. Using ANY battery type, rechargeable or not, carries some risk. I make no claims otherwise, or to being right in my opinions. All risk on you here, so take it or leave it where angles might fear to tread...
[/size]
Re-posts from elsewhere...
Don't know what might be lurking around in the Tesoros, but in terms of less voltage giving less depth, most if not all modern detectors use a voltage regulator to keep a constant static stable voltage to the circuit board. Meaning, as the battery drains from say 12V down to 11 or lower, it won't make a lick of difference as to what the circuit board sees. If the voltage regulator is designed for say an 8 or 9V output, then that's what the board sees no matter how much voltage you feed the regulator.
And, if the voltage is too high, it can cause the regulator to either burn out or shut down on a thermal overload until it cools off (if it has one). Reason why the output voltage is kept constant and unchanging, is that if it was constantly seeing a moving/charging/draining voltage, the detector would be drifting and becoming unstable, or at the very least would require some kind of constant user adjustment to stabilize things. It's not like a flashlight, where as the battery drains the bulb gets less bright.
So long as the source voltage (battery) is above the minimum required input to the regulator, then most if not all modern detectors are going to get the same amount of depth when the battery is new versus when it's getting near the low battery alarm. I've played with linear regulators in projects, and the required minimum input voltage is usually say 1 or so volts above the static voltage. In other words, if it's putting out 9V, then the minimum input voltage might be around 10V, where when it drops to around that, the low battery alarm kicks in.
I've seen people try to increase the depth of a modern machine by running more voltage to it. If there is a regulator between the battery and the circuit board, all they are doing is risking frying the regulator. They usually have a rather wide input range they can handle, or at least the ones I've played with in the past did, but there are limits. Linear regulators, for one, bleed off excess voltage buy shunting it off as heat to a heat sink on the top of them.
Should the heat generated by too excessive voltage be beyond what the heat shrink can bleed off as it runs, then the ones I've messed with have an internal thermal overload that will shut it down. Once cooled off, if your lucky it'll work again, but the overloads are meant as a safety and not a switch intended to be relied on, as they have a limited life span and might fail in 1 trip or might fail in 20. You just never know, because they were never intended to be used.
Alkalines are usually the type that are needed to be used in detectors. Those cheaper non-alkaline types are probably a bad move unless a manual says they are OK to use. Rechargeables are a far better option to save money these days. Needless to be throwing out batteries and money with it, when a good nimh will give you hundreds of cycles if not more before it needs replaced.
Used to be years ago Nimhs and nicads had very low capacity compared to what you can get them in these days. Higher capacity= more run time, but with the lower capacities of years ago, people to this day believe they don't offer much by way of run times. Truth is a good 2500ma or higher nimh AA cell will often out run a non-rechargeable AA in run times, or at least they have for me when I've timed it in various devices, such as a pocket camcorder. Another reason why people feel/felt nimhs or nicads don't offer long run times, is that they will self discharge while sitting on the shelf. Charge it completely and days or weeks later it's pretty well drained again even if not even used.
There are now cheap LSD (low self discharge) nimhs on the market that will hold roughly I think 80% of their charger near a year later. Make sure though that you do your research, as some do not live up to their claims nearly as well as others. If using regular non-LSD nimhs or nicads, be sure to re-peak the pack before a hunt if it's sat for say a week or perhaps two or so. I've got some non-LSD nimhs that will go dead in just a couple of days, while others are good a month or two later maybe, still holding enough charge to run something a good bit.
I haven't kept up on the latest in some lithium based cells, but I suspect there are now 9V cells with partial lithium chemistry that might work very well, and even provide much higher capacities than a nimh can in that size 9V casing, as lithium provides higher capacity, less weight, and in a smaller package. But if you do go that route, be sure the minimum voltage drain allowed of the cell, if it has one, is above where the low battery alarm kicks in on your machine.
I run a 3 cell lipo in my machine, to replace 8AAs, which is roughly the same capacity as the stock nimh pack but not nearly as heavy, and the 3 cell pack is very tiny compared to 8AAs. I wired a small plug into my alkaline holder to plug it into, yet I can still move that tiny plug out of the way and run 8AAs again in there if I ever wanted to for some insane reason. Lipos charge in a hour without stressing the pack, but again there is a minimum drain issue- Lipos should never be drained past 3V per cell, for a total of 9V in series. Luckily my machine sounds it's low battery alarm at around 10V, so I'm good. Short of keeping track of hours used and checking to establish the max you should risk on that via a volt meter, there are little cheap $3 lipo alarms you can plug into the pack that will sound off once it reaches say 9V for the minimum drain down on a 3 cell series pack (3V per cell X 3 in this case).
The fully charged voltage is 12.6V, so right around what a fresh 8AA series pack of regular batteries would be. Another perk to lipos, like many lithium based cells, is that they have just about zero discharge sitting on the shelf. Charge it full, and months later it's ready to rock and roll, but it's not healthy to store them at full charge, so ones I don't plan to use for long periods I put at a specific sleep voltage and store in a fire proof box in my refrigerator.
There are also LifePo4 cells out now that look interesting. The voltage is higher though than an AA, so though they are AA in shape, you can't cram 8 of them into a holder, so they often come with 4 dummy cells to use with them to cut the voltage in half. But, that voltage might still be a bit too high for some machines to handle, so use caution. The minimum voltage drain of those cells would also have to be looked into to make sure your low battery alarm will sound before hand, or by way of keeping track of hours used and then checking with a volt meter to judge cut off points where you should re-charge.
I may look further into what the latest and greatest in 9V lithium based cells might be out there, just to throw one in my Pro Pointer, although that thing gives me almost a year's run time anyway so I never felt the need to use a rechargeable in it.
Final thought- Consider, if you want to stick with non-rechargeables, perhaps the lithium based Energizers. Not sure if they sell those in a 9V but I would suspect they do. I have used them, giving to me from somebody, in various things and must say the extra cost is worth the extremely longer run time they give. They are also very light. 8 of them in an AA holder is a good bit less weight than 8 regular non-rechargeables, like most lithiums are. While these AA non-rechargeable Energizer lithium based cells are light, Lipos have a soft outer casing, so in terms of the weight factor it don't get much lighter than those. For that reason they are big in RC electric plane applications. Smaller, lighter, and higher capacity than other battery types, and able to deliver some rather high current demands.
Battery/charger technology in detecting circles seems to lag about 20 years behind, including price. One of my hobbies is building RC electric planes running brushless motors and lipo batteries, as well as using nimh or nicad packs for transmitters and other various projects in the past. In electric RC planes, knowing the particulars of your chargers and batteries can mean the difference between making it back with the plane or losing power at the wrong moment. That's why knowing batteries/chargers is often a passion of guys into RC stuff. I'm just one of those freaks in that respect.
There are some very cheap alternatives these days out there in both batteries and chargers, usually places that cater to the RC crowd, where the latest and greatest in batteries or chargers are dirt cheap if you know where to look.
One of my biggest complaints about the chargers and batteries that come with many detectors are two things- First, the "charger" isn't really a charger, but rather a simply transformer putting out a trickle charging rate. By keeping the current low, they are eliminating the need to supply a true charger that can tell when the pack has peaked and terminate the charge. While this is said to be OK for nicads (feeding them a trickle charge even though they've peaked), I've always heard this was harmful to nimhs and would lesson their life and capacity over time.
Using a ultra slow trickle current also means very long charge times before the pack is fully peaked. Senseless to charge a pack that slow. Beyond what is needed for the health of the pack. Many nimhs/nicads these days can handle 1C charge rates (1 hour charge rates), or even if not that charging at say 1/3rd to 1/2 the C (capacity) of the pack in terms of current being used is not going to be harsh to it. So long as the pack isn't getting hot it's not being pushed. Heat destroys batteries. Luke warm is OK.
The other complaint I have is they often use very low capacity nimh or nicad cells compared to what is out there these days. Again, about 20+ years behind the times. Typical capacity in stock nimhs might be around 800 to 1500 (if your lucky) MA. These days there are dirt cheap nimhs in the 2600ma or even higher range. About double the run time at least of a stock pack.
No, there simply isn't any reason for wasting money on non-rechargeables in this day and age. Shorter run times with rechargeables? Nope, if you buy the right capacity. Loss of performance or depth of the detector? Again, nope. Cost? Rechargeables/chargers are dirt cheap if you know where to look. Long charge times? Nope again...You can charge a pack in an hour, or say 2 or 3 if you don't want to push things, and even then these are charge times for a completely dead pack and not one still somewhat charged, in which case the charge time will be much lower.
Only thing I'd suggest, is that if buying a charger get one that displays the capacity put back into the pack. This allows you to monitor the health of the pack over time. If the pack starts to show less capacity put in from a completely dead starting point to contrast, then charge and drain the pack (car tail light bulb or something) about 3 to 5 times in a row to exercise it. Some believe only nicads benefit from this, but the truth is even nimhs will give much more capacity if they are exercised say 2 or 3 times a year by 3 to 5 drains/chargers in a row. With a charger that shows capacity put back in you can see the increase in capacity with each re-charge doing that, and then stop when after say 2 chargers in a row you really aren't seeing any more increase.
Also, when shopping for a charger, make sure it does the cell type you are using (nimhs/nicads), and also that it will do up to at least the number of cells you are charging (say 8 for some detectors), and finally that the output current, if you can't change it, is no higher than what the cells recommend (probably about 1/3 on up to 1C max). Many chargers don't come with individual holders for each cell. Easy enough to pick that up at Radio Shack for about $2. Simply get a series holder for say 8 AAs, hook that up to the charger, and the charger will charge all 8 cells at once (if it can do up to 8 cells...read it's specs).
If you meant that a rechargeable nimh will hold it's voltage very steady for more of it's discharge span, yep...They may start out at a lower voltage but will flat line that voltage to near the end, while a non-rechargeable will drop it's voltage as it drains much faster. The lipo I use in my machine also holds it's voltage very high as it drains, and in that case even though I used a smaller capacity pack than the stock nimh pack, the lipo gives me equal or longer run times, since it holds it's voltage even higher than a nimh and keeps it up there higher as it flat lines until near the end.
Faster voltage drop hits the low battery alarm faster, in some respects even with a higher capacity pack, unless the difference in pack sizes is greater than a certain point to make up that difference. At least based on my run time impressions, without timing them to be exact, that's the impression I've had of my 750ma lip 3 cell versus a 1000ma nimh pack. On the other hand, the lipo, like any rechargeable in healthy shape, will hold a good bit higher capacity when charged than the label says. I have a charger that displays this and that's one of the ways I monitor pack health. Of course they have to be drained near the low voltage limit (9V in this case) to see how much capacity is put back in when charged back up.
As for the faster drop off to dead right at the end, yep...rechargeables do that, where as a non-rechargeable will tend to have a more steady rate of decline right near the end, but in some respects the non-rechargeable gets near that end faster with it's more steady drop of the voltage as it drains. Lithium based batteries are even more know for this quick "I'm dead" type of drop off right near the end. You can see this with those Energizer lithium non-rechargeable AAs. If you have a voltage monitor on your detector, I've seen a friend check his for a hunt and it was showing very high voltage still, yet when we got out in the field within under an hour the low battery alarm sounding due to it being near that end from all the prior hunting he did with those cells before hand.
Those Energizer lithium cells are well worth the bit of extra cost for the much longer run times. I haven't crunched the numbers but I bet they more than save you the extra money you spent. I know in a camcorder I used them in they seemed to give me about 3 times the run time of regular batteries in it when filming video. The extra weight savings of them is also well worth the price. Very light. Not sure if Energizer makes lithium 9V non-rechargeables but if they do I'd try those and see how much run time versus cost they have, if you don't want to go the rechargeable nimh or other route...
Final thought on battery safety- Lipos have a bad reputation, but mainly because early on they featured no balance port on the packs or chargers, and so in some instances would overcharge a cell or two and could cause a fire. These days they in some ways could be said to be somewhat safer during charging than other battery types the lack balance ports to monitor each cell individually during the charging process.
Still, all lithium based cell types that I'm aware of, can do some very nasty things if punctured and the lithium exposed to moisture in the air. Even the supposed "safer" lithium ion types were recently yoked from airliners due to fire hazards for whatever problems they were experiencing with them. I suspect constant vibration causing shorts perhaps, but haven't read much into it.
On the other hand, even with other non-lithium based types of batteries it's not a good idea to puncture, short, overcharge, charge at too high (fast) of a amp rate, or over-amp their current delivering abilities. Just like gasoline isn't safe, neither are batteries, but with some simple common sense know-how the risks to handling gasoline can be limited, just like it can with batteries. Know what you are doing and why you are doing it, otherwise far wiser to leave things alone...
[size=x-large]First, the disclaimer- Use any advice at your own risk to person, property, or so on. Using ANY battery type, rechargeable or not, carries some risk. I make no claims otherwise, or to being right in my opinions. All risk on you here, so take it or leave it where angles might fear to tread...
[/size]Re-posts from elsewhere...
Don't know what might be lurking around in the Tesoros, but in terms of less voltage giving less depth, most if not all modern detectors use a voltage regulator to keep a constant static stable voltage to the circuit board. Meaning, as the battery drains from say 12V down to 11 or lower, it won't make a lick of difference as to what the circuit board sees. If the voltage regulator is designed for say an 8 or 9V output, then that's what the board sees no matter how much voltage you feed the regulator.
And, if the voltage is too high, it can cause the regulator to either burn out or shut down on a thermal overload until it cools off (if it has one). Reason why the output voltage is kept constant and unchanging, is that if it was constantly seeing a moving/charging/draining voltage, the detector would be drifting and becoming unstable, or at the very least would require some kind of constant user adjustment to stabilize things. It's not like a flashlight, where as the battery drains the bulb gets less bright.
So long as the source voltage (battery) is above the minimum required input to the regulator, then most if not all modern detectors are going to get the same amount of depth when the battery is new versus when it's getting near the low battery alarm. I've played with linear regulators in projects, and the required minimum input voltage is usually say 1 or so volts above the static voltage. In other words, if it's putting out 9V, then the minimum input voltage might be around 10V, where when it drops to around that, the low battery alarm kicks in.
I've seen people try to increase the depth of a modern machine by running more voltage to it. If there is a regulator between the battery and the circuit board, all they are doing is risking frying the regulator. They usually have a rather wide input range they can handle, or at least the ones I've played with in the past did, but there are limits. Linear regulators, for one, bleed off excess voltage buy shunting it off as heat to a heat sink on the top of them.
Should the heat generated by too excessive voltage be beyond what the heat shrink can bleed off as it runs, then the ones I've messed with have an internal thermal overload that will shut it down. Once cooled off, if your lucky it'll work again, but the overloads are meant as a safety and not a switch intended to be relied on, as they have a limited life span and might fail in 1 trip or might fail in 20. You just never know, because they were never intended to be used.
Alkalines are usually the type that are needed to be used in detectors. Those cheaper non-alkaline types are probably a bad move unless a manual says they are OK to use. Rechargeables are a far better option to save money these days. Needless to be throwing out batteries and money with it, when a good nimh will give you hundreds of cycles if not more before it needs replaced.
Used to be years ago Nimhs and nicads had very low capacity compared to what you can get them in these days. Higher capacity= more run time, but with the lower capacities of years ago, people to this day believe they don't offer much by way of run times. Truth is a good 2500ma or higher nimh AA cell will often out run a non-rechargeable AA in run times, or at least they have for me when I've timed it in various devices, such as a pocket camcorder. Another reason why people feel/felt nimhs or nicads don't offer long run times, is that they will self discharge while sitting on the shelf. Charge it completely and days or weeks later it's pretty well drained again even if not even used.
There are now cheap LSD (low self discharge) nimhs on the market that will hold roughly I think 80% of their charger near a year later. Make sure though that you do your research, as some do not live up to their claims nearly as well as others. If using regular non-LSD nimhs or nicads, be sure to re-peak the pack before a hunt if it's sat for say a week or perhaps two or so. I've got some non-LSD nimhs that will go dead in just a couple of days, while others are good a month or two later maybe, still holding enough charge to run something a good bit.
I haven't kept up on the latest in some lithium based cells, but I suspect there are now 9V cells with partial lithium chemistry that might work very well, and even provide much higher capacities than a nimh can in that size 9V casing, as lithium provides higher capacity, less weight, and in a smaller package. But if you do go that route, be sure the minimum voltage drain allowed of the cell, if it has one, is above where the low battery alarm kicks in on your machine.
I run a 3 cell lipo in my machine, to replace 8AAs, which is roughly the same capacity as the stock nimh pack but not nearly as heavy, and the 3 cell pack is very tiny compared to 8AAs. I wired a small plug into my alkaline holder to plug it into, yet I can still move that tiny plug out of the way and run 8AAs again in there if I ever wanted to for some insane reason. Lipos charge in a hour without stressing the pack, but again there is a minimum drain issue- Lipos should never be drained past 3V per cell, for a total of 9V in series. Luckily my machine sounds it's low battery alarm at around 10V, so I'm good. Short of keeping track of hours used and checking to establish the max you should risk on that via a volt meter, there are little cheap $3 lipo alarms you can plug into the pack that will sound off once it reaches say 9V for the minimum drain down on a 3 cell series pack (3V per cell X 3 in this case).
The fully charged voltage is 12.6V, so right around what a fresh 8AA series pack of regular batteries would be. Another perk to lipos, like many lithium based cells, is that they have just about zero discharge sitting on the shelf. Charge it full, and months later it's ready to rock and roll, but it's not healthy to store them at full charge, so ones I don't plan to use for long periods I put at a specific sleep voltage and store in a fire proof box in my refrigerator.
There are also LifePo4 cells out now that look interesting. The voltage is higher though than an AA, so though they are AA in shape, you can't cram 8 of them into a holder, so they often come with 4 dummy cells to use with them to cut the voltage in half. But, that voltage might still be a bit too high for some machines to handle, so use caution. The minimum voltage drain of those cells would also have to be looked into to make sure your low battery alarm will sound before hand, or by way of keeping track of hours used and then checking with a volt meter to judge cut off points where you should re-charge.
I may look further into what the latest and greatest in 9V lithium based cells might be out there, just to throw one in my Pro Pointer, although that thing gives me almost a year's run time anyway so I never felt the need to use a rechargeable in it.
Final thought- Consider, if you want to stick with non-rechargeables, perhaps the lithium based Energizers. Not sure if they sell those in a 9V but I would suspect they do. I have used them, giving to me from somebody, in various things and must say the extra cost is worth the extremely longer run time they give. They are also very light. 8 of them in an AA holder is a good bit less weight than 8 regular non-rechargeables, like most lithiums are. While these AA non-rechargeable Energizer lithium based cells are light, Lipos have a soft outer casing, so in terms of the weight factor it don't get much lighter than those. For that reason they are big in RC electric plane applications. Smaller, lighter, and higher capacity than other battery types, and able to deliver some rather high current demands.
Battery/charger technology in detecting circles seems to lag about 20 years behind, including price. One of my hobbies is building RC electric planes running brushless motors and lipo batteries, as well as using nimh or nicad packs for transmitters and other various projects in the past. In electric RC planes, knowing the particulars of your chargers and batteries can mean the difference between making it back with the plane or losing power at the wrong moment. That's why knowing batteries/chargers is often a passion of guys into RC stuff. I'm just one of those freaks in that respect.
There are some very cheap alternatives these days out there in both batteries and chargers, usually places that cater to the RC crowd, where the latest and greatest in batteries or chargers are dirt cheap if you know where to look.
One of my biggest complaints about the chargers and batteries that come with many detectors are two things- First, the "charger" isn't really a charger, but rather a simply transformer putting out a trickle charging rate. By keeping the current low, they are eliminating the need to supply a true charger that can tell when the pack has peaked and terminate the charge. While this is said to be OK for nicads (feeding them a trickle charge even though they've peaked), I've always heard this was harmful to nimhs and would lesson their life and capacity over time.
Using a ultra slow trickle current also means very long charge times before the pack is fully peaked. Senseless to charge a pack that slow. Beyond what is needed for the health of the pack. Many nimhs/nicads these days can handle 1C charge rates (1 hour charge rates), or even if not that charging at say 1/3rd to 1/2 the C (capacity) of the pack in terms of current being used is not going to be harsh to it. So long as the pack isn't getting hot it's not being pushed. Heat destroys batteries. Luke warm is OK.
The other complaint I have is they often use very low capacity nimh or nicad cells compared to what is out there these days. Again, about 20+ years behind the times. Typical capacity in stock nimhs might be around 800 to 1500 (if your lucky) MA. These days there are dirt cheap nimhs in the 2600ma or even higher range. About double the run time at least of a stock pack.
No, there simply isn't any reason for wasting money on non-rechargeables in this day and age. Shorter run times with rechargeables? Nope, if you buy the right capacity. Loss of performance or depth of the detector? Again, nope. Cost? Rechargeables/chargers are dirt cheap if you know where to look. Long charge times? Nope again...You can charge a pack in an hour, or say 2 or 3 if you don't want to push things, and even then these are charge times for a completely dead pack and not one still somewhat charged, in which case the charge time will be much lower.
Only thing I'd suggest, is that if buying a charger get one that displays the capacity put back into the pack. This allows you to monitor the health of the pack over time. If the pack starts to show less capacity put in from a completely dead starting point to contrast, then charge and drain the pack (car tail light bulb or something) about 3 to 5 times in a row to exercise it. Some believe only nicads benefit from this, but the truth is even nimhs will give much more capacity if they are exercised say 2 or 3 times a year by 3 to 5 drains/chargers in a row. With a charger that shows capacity put back in you can see the increase in capacity with each re-charge doing that, and then stop when after say 2 chargers in a row you really aren't seeing any more increase.
Also, when shopping for a charger, make sure it does the cell type you are using (nimhs/nicads), and also that it will do up to at least the number of cells you are charging (say 8 for some detectors), and finally that the output current, if you can't change it, is no higher than what the cells recommend (probably about 1/3 on up to 1C max). Many chargers don't come with individual holders for each cell. Easy enough to pick that up at Radio Shack for about $2. Simply get a series holder for say 8 AAs, hook that up to the charger, and the charger will charge all 8 cells at once (if it can do up to 8 cells...read it's specs).
If you meant that a rechargeable nimh will hold it's voltage very steady for more of it's discharge span, yep...They may start out at a lower voltage but will flat line that voltage to near the end, while a non-rechargeable will drop it's voltage as it drains much faster. The lipo I use in my machine also holds it's voltage very high as it drains, and in that case even though I used a smaller capacity pack than the stock nimh pack, the lipo gives me equal or longer run times, since it holds it's voltage even higher than a nimh and keeps it up there higher as it flat lines until near the end.
Faster voltage drop hits the low battery alarm faster, in some respects even with a higher capacity pack, unless the difference in pack sizes is greater than a certain point to make up that difference. At least based on my run time impressions, without timing them to be exact, that's the impression I've had of my 750ma lip 3 cell versus a 1000ma nimh pack. On the other hand, the lipo, like any rechargeable in healthy shape, will hold a good bit higher capacity when charged than the label says. I have a charger that displays this and that's one of the ways I monitor pack health. Of course they have to be drained near the low voltage limit (9V in this case) to see how much capacity is put back in when charged back up.
As for the faster drop off to dead right at the end, yep...rechargeables do that, where as a non-rechargeable will tend to have a more steady rate of decline right near the end, but in some respects the non-rechargeable gets near that end faster with it's more steady drop of the voltage as it drains. Lithium based batteries are even more know for this quick "I'm dead" type of drop off right near the end. You can see this with those Energizer lithium non-rechargeable AAs. If you have a voltage monitor on your detector, I've seen a friend check his for a hunt and it was showing very high voltage still, yet when we got out in the field within under an hour the low battery alarm sounding due to it being near that end from all the prior hunting he did with those cells before hand.
Those Energizer lithium cells are well worth the bit of extra cost for the much longer run times. I haven't crunched the numbers but I bet they more than save you the extra money you spent. I know in a camcorder I used them in they seemed to give me about 3 times the run time of regular batteries in it when filming video. The extra weight savings of them is also well worth the price. Very light. Not sure if Energizer makes lithium 9V non-rechargeables but if they do I'd try those and see how much run time versus cost they have, if you don't want to go the rechargeable nimh or other route...
Final thought on battery safety- Lipos have a bad reputation, but mainly because early on they featured no balance port on the packs or chargers, and so in some instances would overcharge a cell or two and could cause a fire. These days they in some ways could be said to be somewhat safer during charging than other battery types the lack balance ports to monitor each cell individually during the charging process.
Still, all lithium based cell types that I'm aware of, can do some very nasty things if punctured and the lithium exposed to moisture in the air. Even the supposed "safer" lithium ion types were recently yoked from airliners due to fire hazards for whatever problems they were experiencing with them. I suspect constant vibration causing shorts perhaps, but haven't read much into it.
On the other hand, even with other non-lithium based types of batteries it's not a good idea to puncture, short, overcharge, charge at too high (fast) of a amp rate, or over-amp their current delivering abilities. Just like gasoline isn't safe, neither are batteries, but with some simple common sense know-how the risks to handling gasoline can be limited, just like it can with batteries. Know what you are doing and why you are doing it, otherwise far wiser to leave things alone...