Kai TrustFire 14500 current draw

[max52]

I bought 6 Trustfire 14500 to use in modified Fivemega parallel 17650 battery holder to run a philips 5761. These cells are adverised as 'fully protected" , however the only specs advertised is that have 900 Mah. In parallel, in a 3 14500 holder, these cells should be able able to handle the current daw from the 5761.
Am I correct?
I have also noticed that the recent 5761s "insta-flash" as low as 6.9 vBat.
would like like some suggestions

 

[mdocod]

Um... that's a very strange configuration idea there... I can't fathom why you wouldn't just use 6AA NIMH in series in this case, it would be safer, have more capacity, and be MUCH easier to charge.

14500s don't have 900mAH, 700mAH is probably still an overstatement... but lets for the sake of comparison give the cells the benefit of the doubt and say they are capable of delivering 700mAH INTO a heavy load (they won't BTW, just for the sake of comparison lets pretend they are sortof good)

3 cells in parallel would give 2.1AH capacity. So, depending on the brand and manufacture recomendations, the maximum safe current draw from the pack would be between ~3-4.2AMP. The 5761 draws OVER 5 AMPS. So this configuration would not even be safe.

Then there is the issue of charging- you need to find a way to balance the cells when dealing with li-ion cells, which means either removing them from the pack very frequently, or putting a balance charging tap on them. with a NIMH pack, you could just slap it on a series charger and be done with it. You can get plenty of NIMH cells around 2AH that will perform into the load of a 5761

 

[Kevin Tan]

I beg to differ mdocod, agree with the discharge issue and the capacity, but the protected cells will take serial charging as they have individual protection built in.I would advise not to charge serial as the pack as built is setup for serial-parallel. So what charge rate are you gonna use? 700ma x 3 (parallel) x 8.4v ? Imagine the possibility of things going wrong. NOT advisable but is possible.

 

[mdocod]

but the protected cells will take serial charging as they have individual protection built in

protection prevents catastrophic failure and should not be used as a replacement for balance charging. Protection circuits prevent charge voltage from rising above ~4.3V, they will not terminate a cell at 4.200V like they need to be. The lowest voltage I have seen on a PCB for charge protection is 4.25V, AWs and most others are around 4.3V.

The other issue is that more than likely the PCB on those cells is set to cut with any current above ~1.5A (give or take a few hundred mA), so the 5761 is not only out of the equation because of safety, but the cells probably won't do it period.

 

[Illum]

14500s don't have 900mAH, 700mAH is probably still an overstatement...

I dunno about 900mah, but AW protected 14500s are at 750 mah

 

[DM51]

mdocod is right.

1. Under load you will be lucky to get 1800mAh out of 3p 14500s.

2. A Philips 5761 will therefore be FAR too much for these cells - consider a WA1111 or 1274 instead.

3. Don't charge them in series - that is asking for trouble.

 

[max52]

I would not charge as a pack. I would balnce when recharging. I would use 2 Fivemega holders. From article on parallel versus series.

Why parallel?
A parallel arrangement means that current is shared between batteries and this means that each battery is exercised less vigorously which may increase usable capacity.

if 2, 3 x14500 holders where used the current draw would be distributed.

Am I correct?

 

[SilverFox]

Hello Max,

You are correct.

In theory, you are looking at cells that claim to be capable of 900 mAh, but you do not know how they will test out under high loads. If you parallel 3 of those cells, and if they are really good for 900 mAh, you theoretically could be good for 5.4 amps. Using two holders in series will give you twice the voltage of a single holder, but the capacity will not increase.

In theory, you should be good to go.

However, others have pointed out that the 900 mAh claim is most likely inflated. 700 mAh may be a more realistic value to use. This would give you a maximum current of 4.2 amps. Since you need a little over 5 amps, you would be driving your cells slightly above 2C.

When you overdrive cells, the cell heats up and the voltage drops. The heat kills the cells and the lower voltage means your lamp will not be as bright.

I have had cells die in as little as 10 cycles when pushing them to 2C and beyond. This means that the cells dropped to below 80% of their initial capacity.

If you started out with cells that were actually 700 mAh and pushed them hard, they could be down to around 560 mAh in as little as 10 cycles. Now you are running at 1680 mAh and a 5 amp draw would be nearly 3C.

If you are just playing around, don't care if you ruin the cells, take the appropriate safety precautions, and don't let the light be used by "uninformed" people, it would be an interesting exercise. However, if you want a dependable light, you should tone down the current draw, or use batteries with higher capacity to give you a better margin of safety.

Tom

 

[max52]

Thanks for the info. I think I will try the WA1274. Should make a nice light.