NiMH batteries

[Steve Hargreaves - Moderator]

Or indeed just charged as a single pack....??

 

My eldest has a 1/10th scale electric buggy powered by a 3300mAh 6 cell sub C pack of NiMh.....his little brother will get one tomorrow for his birthday....(Ahhhh!!)

 

My question relates to charging the batteries.....as they will obviously want to race together then both batteries are likely to need re-charging at the same time........my charger is cabable of delivering the power to charge both packs but I was wondering if I should charge them in parallel as a 6 cell 6600mAh pack or in series as a 12 cell 3300mAh pack.

 

Or should I just charge the packs individually & separately.....my concern is that they may be in different states of charge when removed from the cars.....

 

Both batteries are the same make & capacity & similar age/usage....

 

Your thoughts as ever would be most welcome....

[Vecchio Austriaco]

I suppose the batteries are of equal capacity.

You have to charge them in series, only in this case they get the same current. (therefore I am asking if they are the same size - with different sizes it will not work...

 

Cheers

 

VA

[Steve Hargreaves - Moderator]

Yes both are the same capacity at 3300mAh......

 

My concern with series charging is just that...they will get the same current regardless of their state of charge.....with parallel I'm thinking the battery with the lowest state of charge will "absorb" the most current......

[PatMc]

Steve,

they should be charged in series.

They will get the same current but the charge will cease when the least discharge battery reaches delta peak so no harm will be done.

I used to do this with similar sized nickel batteries then put each battery back on charge singly. This was to ensure they were topped up but IMO probably not necessary in your case as they won't be that far apart.

[Tbone]

 

Edited By Tbone on 25/11/2011 12:10:08

Edited By Tbone on 25/11/2011 12:13:21

Edited By Tbone on 25/11/2011 12:27:35

Edited By Tbone on 25/11/2011 12:28:22

[Peter Beeney]

Steve - Just to be different, I would personally charge them in parallel, but it probably won’t make much difference. As PatMc says, a series connection is obviously just as good. Try both and see which one suits you best. If you connected one half flat battery in parallel with a totally flat battery and then put them on charge the flat battery would first charge up to the state of charge of the other and then they would both charge together. The voltage on one pack cannot rise above the voltage of the other. The ubiquitous Kirchoff’s Voltage Laws see to that. (He’s always lurking in the background, there’s no escape!) So that says there’s a good chance they would terminate more or less together. You can easily try it, just watch it with a voltmeter. Give them a long 0.1C equalising charge occasionally. I used 3 large nicads in parallel as a glow driver for several years, and they got some real abuse on occasions but they never complained; and they always terminated a fast charge on peak detect ok.

Another alternative is a charger with two outputs, or even three, so then there is never any need consider how you are going to charge. But it does need to be hanging on a pretty hefty supply, it can consume the watts big time!

There as always been the story that you can’t charge nickel cells in parallel, and to some extent I think there might be some justification in that, but Skyleader always charged their transmitter packs in parallel. I wondered why this was so in the beginning, but their charger was a multi function type, mains driven but also a 12 volt field charger; one tx and two rx output. All 50 mA output I suspect, because all the batteries were 500 mAh nicads anyway. The tx had two 4 cell packs, in series for 9.6V when it was switched on. When it was switched off, however, the wiring was arranged so that the pack became a centre tapped battery and the charging lead was a three wire to a DIN charging plug on the box. I reckoned the reason for this is because you can’t properly charge a 9.6V nicad directly from a 12 volt source, the car battery at-the-field charger, especially if the circuit goes via a PN junction or two, the supply voltage is not high enough. There was no inverter in the charger to raise the voltage. I’ve never really thought about this until now, but I reckon there might have been just two 4.8 chargers, hence two rx outputs, and so that when you were charging a tx and rx those batteries were also in parallel.
As far as I’m aware, this worked successfully for many years, or at least it did for me anyway.

Once upon a time a young lad lived opposite me and he had an r/c electric buggy. He was often knocking on the door in the evenings for some help with the latest breakage. This was nicad powered, with a slow mains charger, on all night stuff. So I cobbled a very primitive fast charger together, just a big resistor on a piece of aluminium sheet, his dad, a builder, found a 12 volt car battery and he had a charger in his garage so it was permanently on charge; and his mum found an old alarm clock so that he could time the charge. I think we calibrated it by seeing how long it took to get hot from flat, about twenty minutes as I remember. But the lad, Jamie, always said to me the the best performance was from the first drive in the morning, after it had been on charge all night. This is perhaps not a general rule, for a number of reasons, but I’ve never seen any evidence that a slow charge is at least detrimental to a fast charge.

You now need one for yourself, so that your lads have always got someone they can run rings round……

PB

[Steve Hargreaves - Moderator]

I always miss the third dimension when I'm driving the boys' cars & boats......flying models are much more interesting.....

 

The boys run rings around me all the time anyway.....with or without their cars!!!!

 

Thanks for the reply Peter.......that's how I had seen it working too. My concern with series charging was that one battery might be full but the peak detect wouldn't kick in.....charging in parallel should bring both batteries to the same state of charge.....

[PatMc]

Charging in series isn't "just as good", it's much better & safer. Charging nickel batteries in parallel is a thoroughly bad idea & it won't bring them to the same state of charge. The delta peak will most likely be missed for both batteries leading to them both being severely overcharged. If unattended overheating this could result in severe overheating resulting in some electrolyte venting off causing permanent reduction in capacity or worse one or more cells could explode.

 

If your charger can deliver about 7A & handle 120W you should be able to charge in series within 30 mins. To charge in parallel in 30 mins it would need to handle 14A @ 120W.

[Peter Beeney]

Yes, I should have added my usual addition to my post to say that this was not any form of instruction or advice on how to do it. It would simply be my own choice, if I wished so to do.

With regard to the same state of charge in parallel, as I said, the two 4 cell packs in both my Skyleader transmitters were always charged in parallel, albeit at a slow rate, of course. Back in those days I did a lot of slope and thermal soaring, and I always felt I had up to around four and a half hours plus worth of sensible air time. Given that the cells were 500 mAh capacity and the tx power requirement must have been in the order of toward 100 mA this would seem to be about right. I never ever felt that the battery power was inadequate because of insufficient charging.
When I did some experiments with fast parallel charging I was slightly sceptical that it was not a good idea so I was quite pleasantly surprised to find that it actually worked quite well. Faultlessly, in fact. I monitored the set up carefully and at no time did it ever give me any cause to suspect that it was going to noticeably misbehave itself; and as I said, I used 3 cells in parallel as a 1.2V glow source for a long time. They always charged up just fine.

… But there are chargers… … and then there are chargers! Being familiar with them all is not one of my strong points, I’m afraid. I’m not even sure that they are all even capable of charging and discharging one cell on it’s own. So that might rule out single cell working for some modellers. As it happens, my charger has three outputs so I would seldom need to charge cells in parallel anyway.

A long time ago, when fast charging of modelling batteries was just beginning to happen, I was browsing in a far away model shop when I heard the proprietor graphically explaining to another customer how a modeller had had his hands blown off in a flight battery explosion and also how all the glass had been blown away in a similar incident in a modellers estate car. The gentleman was very keen on getting fast charging banned, it seemed. This intrigued me, so I spent some time actually blowing some cells apart, three in total. This actually proved to be quite difficult, a full short would not do it on discharging; and I couldn’t get the current necessary flowing in sufficient quantity when charging so I used a straight connection on to a fully charged 12 volt car battery. I also had to use car-start jump leads, to keep the total impedance low enough. The cells simply split open, with a small popping noise; I thought this was rather more damp squib than rook scarer. I do emphasise however, this was just how I found it happened, this definitely does not mean that I’m saying that cells cannot and will not explode in a much more violent form.

I’ve long ceased to be surprised at just how far aeromodellers can sometimes stray from the beaten track. I’ve seen at least one flight pack that got so hot (in flight) that what was left of the heatshrink had burnt itself onto, and had literally become part of, one cell casing. I therefore think it’s quite possible this pack may have been close to blowing up at the time, who knows.

I will try and make an effort to lash something together and have another dabble at a spot of parallel charging. I will document the result, whatever happens, and post it here. Then it can be dissected and kicked to death as much as anyone chooses to do, if anyone would be sufficiently interested, that is.

And, once again, this should not be construed as any semblance or shape of advice or instruction on how charge or discharge any battery. This is just a narrative on what I’ve noticed by observation and practical experience.

PB

[Steve Hargreaves - Moderator]

What PatMc says is interesting & makes sense too.....

 

After all the cells within the battery are all in series.......

 

What to do???

[Bladerunner]

Posted by Steve Hargreaves on 25/11/2011 11:17:26:

Or indeed just charged as a single pack....??

 

My eldest has a 1/10th scale electric buggy powered by a 3300mAh 6 cell sub C pack of NiMh.....his little brother will get one tomorrow for his birthday....(Ahhhh!!)

 

My question relates to charging the batteries.....as they will obviously want to race together then both batteries are likely to need re-charging at the same time........my charger is cabable of delivering the power to charge both packs but I was wondering if I should charge them in parallel as a 6 cell 6600mAh pack or in series as a 12 cell 3300mAh pack.

 

Or should I just charge the packs individually & separately.....my concern is that they may be in different states of charge when removed from the cars.....

 

Both batteries are the same make & capacity & similar age/usage....

 

Your thoughts as ever would be most welcome....

 

You should charge NiCd and NiMh chemistry cells/batteries in series because they have a negative delta-V(voltage reduction) once fully charged; if you charged them in parallel this would result in one cell or battery "hogging" all the charge current as its voltage reduces relative to the other. When fast charging it is especially important to charge in series as the effect is more pronounced and it could lead to overheating and cell damage.

Slow charging at a current rate of 0.1C(1/10th of battery capacity) for 14Hrs to 16Hrs will generally result in a fully charged and balanced pack. In this instance the cells are able to cope with the relatively low heat dissipation which results once fully charged, allowing all the cells to fully charge without damage.

 

Ideally follow the manufacturers recommendations. Be aware that fast charging in series could lead to overheating and damage anyway if the cells happen to be significantly unbalanced. Cells in a battery pack should be of the same type and capacity. Never leave charging batteries unattended, check them periodically.

 

Regards.