advertisement


Question about charging PP3 batteries (Voltage)

Johnny Blue

I made it to 'Member'!
Sorry, chaps, this is really small fry for the DIY forum, but don't where else to ask...

I've got some rechargeable 9V PP3 NiMH batteries (200mAh) and have just discovered that my (rather ancient) battery charger (or at least the only one I have with the appropriate PP3 contacts) has a charging voltage of 7.2V. Is this is a problem, i.e., will the 7.2V charger charge 9V batteries? Should I allow extra time to compensate?

(I realise the charger is only 7.2V because it dates from the days before NiMH had even been thought of -- it's almost 30 years old! -- and was designed only for NiCd PP3s, which presumably only ran at 7.2V.)
 
Stand down, men, I think I've answered my own question...

Further research on rechargeable PP3s reveals that my 9V NiMH are probably only 7.2V or (at most) 8.4V in any case, since they will contain either 6 or 7 1.2V cells, so the charger I've got should be fine.
 
Well, I just measured a NiMh PP3 type battery (200mAh) that was charged a day ago and it reads 9.4V open circuit. This is a new battery bought from Lidl just a few days ago.

The charger I'm using surprisingly, has an open circuit voltage of just over 15 V. It has NimH and NiCd charging settings.

I thought that ideally, different charging characteristics were needed for NiMh and NiCd batteries, but some chargers have no adjustment.

Indeed, I bought some AA/AAA USB chargers from Poundland (of course £1 each) and they have no adjustment for NiMh/ NICd but the instructions say suitable for both. They seem to work with both too.
 
Thanks for that, very useful (and my PP3s are also from Lidl!).

I've got another (slightly more modern!) charger that is switchable from NiMH to NiCd, and I think the only difference is charging time, as the former generally have much larger capacities than the latter. (And, in any case, the charge times are geared to whatever was the maximum capacity of batteries when the charger was manufactured: I have to charge my 2500mAh batteries twice over to get them fully charged, as my charger switches off after 8 hours, which only delivers 1200mAh.)
 
The NiMh batteries from Lidl seem very good value and are quite high capacity (AA 2500 mAh, AAA 900mAh). Think these were just under £2.50 for four. The NiMh batteries from Poundland are even cheaper, but much lower capacity (AAA 350mAh).
 
The NiMh batteries from Lidl seem very good value and are quite high capacity (AA 2500 mAh, AAA 900mAh). Think these were just under £2.50 for four. The NiMh batteries from Poundland are even cheaper, but much lower capacity (AAA 350mAh).

I used the rechargeable batteries from Lidl for quite a while and they were mostly OK but did vary in how well they lasted or kept their charge. However, I would say that buying Energizer rechargeables is worth the extra cost, and they may even work out less expensive in the long-term!

Last year, my digi camera would hardly function outside at all in the very cold weather with a freshly-charged set of the Lidl batteries but this year it has worked flawlessly with partly dischaged Energizers! ;)
 
I used the rechargeable batteries from Lidl for quite a while and they were mostly OK but did vary in how well they lasted or kept their charge. However, I would say that buying Energizer rechargeables is worth the extra cost, and they may even work out less expensive in the long-term!

Last year, my digi camera would hardly function outside at all in the very cold weather with a freshly-charged set of the Lidl batteries but this year it has worked flawlessly with partly dischaged Energizers! ;)

I think it depends on which batteries purchased at Lidl, the current crop seem to be much better. The ones I bought some time ago were never much good.
 
Well so far the "new crop" 2500 mAh AA batteries from Lidl are doing OK. I'm addicted to LBC radio and listen mainly on a pocket Sony which uses 2xAA. The Lidl ones are doing much better than some Uniross (which is to be expected as they are a greater capacity).

I am also using the Lidl AAs in a Fuji camera and they are OK in that.

Still, time will tell!
 
This being in the diy section .....

I looked into recharging MNiH batteries some months ago because I was using 3 batteries and most chargers require you to charge them in pairs (in series).
Its actually very straightforward to build you own charger, just use an LM317 and a resistor (*1) in (constant) current regulator mode (see the LM317 datasheet). In this mode, the regulator adjusts its output voltage so as to keep a constant current flowing through the resistor (and target circuit).

The resistor determines the current which will be 1.25v/Res
A good value for a PP3 would be 75Ohms = 1.25/75 = 17mAmps
This will slow-charge the battery indefinitely. You can use a higher charging current (lower value resistor) but, when the battery is fully charged, the current will be converted into heat and will destroy the battery :eek:
For AA batteries of 2500mAh, I use a 4R7Ohms resistor = 1.25/4.7 = 265mAmps.
At this higher current, suggest some form of heatsink on LM317 (TO220 version).
When the batteries get warm, they are charged.

I would suggest that the resistor value chosen needs to supply a current which is approx 1/10th of the mAh value of the battery (nb I'm no expert so don't blame me if you burn your house down tho' :eek: )

I knocked up a simple charger which can independently charge between 1 & 4 AA MNiH batteries.

HTH all you diy'ers

*1 You will also need a suitable wallwart to supply the power.
 
Thinks (after reading MikeInUK's post): "I knew I shouldn't have posted in the DIY section"!

On the cheap-as-chips-from-Lidl front: like others here, I have found that one or two from my first batch of 12 don't hold a charge for very long, I hope the second lot I bought more recently (including the PP3s) fare better.
 
NiMH batteries have a typical voltage of 1.2V per cell during discharge, but typically reach about 1.6V per cell by end of charge. As stated in a previous post, charge them with a constant current source that allows them to reach at least 1.6V per cell if you want them fully charged. An EOC voltage of only 1.2V will not do much in most cases.

As rule of thumb, charging NiMH batteries at C/14 (e.g 100mA for a 1.4Ah battery), you don't need to terminate charging automatically (it's not particularly good for them, but it won't hurt too much). At higher rates, charging should be terminated using either voltage-, temperature-, or time-based methods. In this case it's best to use a "proper" charger IC from the good people at Texas Instruments (who, incidentally, pay my wages).
 
Thinks (after reading MikeInUK's post): "I knew I shouldn't have posted in the DIY section"!

On the cheap-as-chips-from-Lidl front: like others here, I have found that one or two from my first batch of 12 don't hold a charge for very long, I hope the second lot I bought more recently (including the PP3s) fare better.

What's "very long"? NiMH batteries have a reasonably high self-leakage (I seem to remember 30% per month, but don't quote me on that).

And brand new batteries may benefit from few charge-discharge cycles before you obtain their rated capacity. Charge them fully and discharge completely a few times and they should improve.
 
What's "very long"? NiMH batteries have a reasonably high self-leakage (I seem to remember 30% per month, but don't quote me on that).

Sorry, I wasn't very clear: what I've noticed is that in a batch of, say, four batteries being used in, say, a radio, I'll find that when they run out, and I check them for charge, one out of the four will be totally flat, and the others still have loads of ooomph!

That one gets labelled 'duff', and although I'll still use it, I know that the others will keep going for much longer, if I use another battery to replace the 'duff' one. I've found the same with NiCds, in fact, so much so, that a 'duff' one won't hold a charge at all and only gets used to make up the 'four' that are required by my chargers.

(Excuse use of technical language: I knew the DIY section was out of my depth!)
 


advertisement


Back
Top