How to charge NiMh batteries

[Rolo Rolo]

This guide deals with a lot of the maths behind charging NiMH, so if that’s not for you, switch off now. It doesn’t deal with NiCad, Li-Ion or anything else.

 

EDIT: Crikey, there's a lot here isn't there. Before you start reading, have the following information to hand.

1) Your battery's voltage (V).

2) Your battery's output rating (mA/h). It could be 1100mA/h, 2200mA/s, yourbattery's#mA/h but expect to see a number in the 1000's for an airsoft battery.

3) Your charger's output in milliamps (mA) for the voltage of the battery you are charging. WARNING: Your charger may not quote an output current for the exact voltage you wish to charge. If it's a cheap charger, watch out. Output current goes down as voltage goes up and up as voltage goes down in low quality chargers.

 

Your battery's rating is it's capacity. The nomenclature for this is C. For example, my battery has a 3300mA/h rating, (theoretically it can discharge 3300milliamps in an hour), so 3300 is to me, C. My friend has an 1150 mA/h battery, her "C" is 1150.

 

IMPORTANT FACT: This mA/h figure relates to a single cell of your battery. NiMh battery cells give out approx 1.2 volts. You can't buy a single 8.4 volt NiMh battery cell. What you have to do (and usually the manufacturers do this for us airsofters) is to is link them together in series and add the voltages up. So...

 

1 cell = 1.2v

2 cells = 2.4v

3 cells = 3.6v

4 cells = 4.8v

5 cells = 6v

6 cells = 7.2v

7 cells = 8.4v

8 cells = 9.6v

9 cells = 10.8v

10 cells = 12v

etc..

This is my original battery. Each cell is 1.2v 3300mA/h and there are seven of them. It's an 8.4 volt battery.

This is my new battery. Each cell is 1.2v 3300mA/h and there are eight of them. It's an 9.6 volt battery.

 

You get the idea? The higher the voltage, the more cells you need. And high voltage doesn't mean high capacity. You can have a 9.6v 1500mA/h battery and a beefier 6v 5000mA/H battery.

 

Okay, there are several ways you can charge a NiMh battery: Trickle, slow, rapid, fast and ultra-fast.

 

(Remember that rating "C", I'm going to refer to it a lot!)

 

TRICKLE CHARGING

NiMh batteries lose their charge with time. Typically a good one will lose 1% a day. A poor-quality or poor condition battery may well lose more. (Actually there are some which DON'T lose their charge at 1% a day, which complicates things a bit, but they are not the norm.) Trickle charge is basically used to counteract this loss and keep the battery fully charged, NOT to charge a battery that is depleted. The charging rate for trickle charging is around C/40. So for my battery that's 3300/40 which is a charging rate of 82.5mA (at 8.4 volts).

IMPORTANT FACT: You need to match your charger to your battery!!!

If I use my battery's 82.5mA trickle charge rate on my friend's 1150mA/h battery, I'll be charging hers at C/14, which is not trickle-charging. It is pretty much.....

 

SLOW CHARGING

Slow charging is how NiMh batteries love to be charged, but as its name suggests it is slow. The charge rate for slow charging is C/10, or 10% of the battery’s capacity. So for me, that’s 3300/10=330mA. But for how long? Batteries are not like a glass of water. You can’t put some in and get out everything you put in. Due to losses you have to put in 150% of what you expect get out of a NiMh battery. (Now you know why people say "You have to times it by 1.5). So as well as the 10 hours I need to charge just to match the output (330mA * 10 hours = 3300mA/h) I also need to charge for another 5 hours to bring me up to the 150%. So that’s 15 hours given a perfect C/10 charge rate.

But in the real world, you get a charger with a rating. This is the info I got with my charger.

 

So let's translate that into use. It's 200mA at 8.4v. So that's it's output for my battery. (the higher the voltage, the lower the mA in cheap chargers). As I explained earlier, I need 330mA/h for a 10% slow charge, so I only have a charge rate of 6% or 0.06C with this 200mA/h charger. This means I have to charge for 3300mA/h / 200mA = 16.5 hours PLUS the extra 50% to counteract the losses, which ends up at 24 hours 45 minutes.

 

Why the hell would anyone want to slow charge? Well there’s at least one good reason. As batteries finish their charge and start to overcharge, they start releasing gasses internally. Most NiMh cells have an inbuilt ability to reabsorb these gasses so long as they do not overwhelm that ability either by too high a charge current or overcharging for too long. Guess what their ability is designed to be able to deal with? You got it! A charge rate of about 10%. More than that and they overheat and start to get damaged. This is why most OEM chargers are slow chargers. It means the batteries can be moderately overcharged without much risk of battery damage and subsequent customer anger with the shouting and biting etc. It also explains why you get pretty vague charging advice with your slow charger. “Until it gets too hot!” seems to be a common theme, and it’s pretty much the truth. Now you know why. But I am old and haven’t got the time to slow charge, which brings me on to…..

 

RAPID CHARGING

Rapid charging is seen as typically 0.3C – 0.5C., or 30% to 50% of the battery’s capacity. Maybe 60% too. The same maths applies, but now things are getting more manageable. If I charge at 0.5C, that’s 3300/2=1650mA for 2 hours plus the 50% extra for losses is 3 hours.

USEFUL FACT: Have you noticed how the charge time is simply the charge rate denominator *1.5? Just a thought….

Well I can live with 3 hours charge time, so I’ve ordered a rapid charger which is switchable between 1000/2000mA.

 

The bad news is that the nice round theoretical 1C and 0.1C numbers have to go out of the window and I deal with the 1000/2000mA rates what I’ve got. The good news is that this is a quality charger and can deliver 1000/2000mA at 6-12v, so that's nice.

 

To work out the charge rate of my charger, I divide the charger’s outputs by my battery rating. Which is 1000/3300=0.3C and 2000/3300=0.61C. Okay, those are nice rapid charge rates so I've got the right charger for my battery if I want to rapid-charge.

 

To work out my charge TIMES, I divide my battery rating by the charger ratings and multiply the number by 1.5 to add in the 50% losses charge.

 

So that’s (3300/1000)*1.5=4h57m and (3300/2000)*1.5=2h28m. I can live with that.

 

At this charge rate, I’ll need to make sure I charge for no longer than I’m supposed to or I’ll cook my battery. I’ll would have needed a timer plug set to the correct time, but I’ve ordered a smart charger which detects the little electrical twitch in the NiMh cell when it’s fully charged and changes from RAPID to TRICKLE charge. (Incidentally, it would be nice if my "smart" charger also had a temperature probe to prevent overheating, but it doesn't. If you're buying a smart charger, one with a temperature probe is a very safe bet, as just sometimes, the little electrical twitch in the battery is not detected at the lower end of the range. With that in mind, I'll always be charging mine at 2000mA. This forces a larger voltage "twitch" when fully charged and should make sure the detection circuitry works reliably and the charger switches to trickle charge when it should.)

But, maybe you want your battery fully charged as fast as possible without risking damage to your battery?! Which brings me to….

 

FAST CHARGING

Fast charging is basically charging at 1C. So my 3300mA/H battery will take a charge of 3300mA for 1 hour, plus the 50% which takes it up to 1h30m. Likewise my friend’s 1150mA/H battery will take a 1C charge of 1150mA for 1.5 hours.

USEFUL FACT: See how that charge rate *1.5 is still working at 1C?

IMPORTANT FACT: Note how with my charger, the RAPID charge rate of 0.3C on my 1000mA output for my 3300mA/H battery becomes a FAST charge rate of 0.87C on my friend’s 1150mA/h battery. And also note how the RAPID charge rate of 0.61C on the 2000mA charger output for my 3300mA/h battery becomes a 1.74C charge rate on her 1150mA/H battery? Which brings me to….

 

ULTRA FAST CHARGING

Ultra fast charging is a charge rate of 1C-10C. At this charge rate, you are putting quite some stress on your battery, so you’ll need to give it some TLC in the form of a slow charge every now and then. It can be done but do I want to put around 30 amps into my battery to charge it at 10C? I’ll pass thanks.

 

CONCLUSION

Remember, a NiMh is not for life. You will probably get well under 1000 charge cycles, so don’t think that you’ll get to keep your batteries in perfect condition forever if you only ever slow charge. It won’t happen. Like us, batteries all get old and die, sooner or later.

 

There are other issues, such as crystalisation, but it’s not a big issue. The bottom line is to slow charge now and then, but rapid or fast charging as the norm is perfectly fine, and ultra-fast charging is possible with care. But to charge your battery properly, YOU MUST KNOW THE RATING OF BOTH YOUR BATTERY AND CHARGER.

 

That’s all folks.