Advice on building a battery pack using 26650 cells

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rufes1
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Advice on building a battery pack using 26650 cells

Hi guys and gals,

I would like to build a battery pack using 4 × 26650 cells in series (14.8v) to power a cordless drill. Now I was going to use TrustFire 5000mah protected cells from Manafont. They seem to be a pretty decent cell.

My number 1 issue is that the cells will not fit into the original battery pack case, so I want to make my own pack by creating a mold, then placing the 4 cells that have been soldered together in series into the mold then pouring resin into the mold to create a solid battery pack.

The thing I am worried about is the heat dissipation, as the solid resin pack will tend to insulate the the batteries and retain heat. I am still not 100% sure what the discharge current will be, but as an estimate say max of 5 amps (probably less but adding a safety factor). Charging will be done much less at around 1 amp.

What are your thoughts? how hot is the pack likely to get with a constant discharge of up to 5amps? is this going to be dangerous?

Thanks!
Matt

willie
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The Li-ion tool packs I have examined had IMR cells with high discharge rates.

rufes1
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Well at the moment the drill uses shitty 1300mah nicad sub c cells connected in series. Obviously I will be using a lithium hobby charger to charge the pack not the original one supplied with the drill.

Shadowww
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You still should consider using IMR cells + BMS instead.
And you’d need 5 of them to replace original 18V NiCad pack, not 4. (5 * 3.7V = 18.5V, way closer to 18V than 4*3.7=14.8V).

rufes1
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Currently its a 14.4v pack running 12 × 1.2v Nicads, so 4 × 3.7 li-ion running 14.8v should be ok. Definitely happy to consider IMR but they will need to be protected as running in series – Not familiar with the term BMS?

Thanks for your help! Smile

Shadowww
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BMS is battery management system, it’s essentially something like protection circuit (+ cell balancer), but for whole pack, not for individual cells.
You can get high-current IMR BMS’es from BatterySpace.
http://www.batteryspace.com/pcmwithequilibriumfunctionandfuelgaugefor148... – here’s one with 30A limit, for example.
And another one: http://www.batteryspace.com/PCM-with-Equilibrium-function-for-14.8V-Li-I...
I guess it’s balancing functionality won’t be important for you as you’ll be using a hobby charger, but protection is quite important.

rufes1
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Hmm, with IMR + BMS the price is getting pretty up there. I spotted this on the website and thought it didnt look too bad.
14.8v 4Ah max 10A discharge…

http://www.batteryspace.com/customizehighpowerpolymerli-ionpack148v4ah59...

Thoughts?

Shadowww
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Well, 10A can get ‘kicked out’ by cordless drill – their inrush current can be 2-3 times bigger than that. Otherwise that pack is fine.

rufes1
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yeh, thats what I am worried about… because the original cells are only 1300mah I thought it couldn’t draw too much current, but like you said its that initial inrush that will make the protection circuit kick in…

Arrgghh too hard!

texaspyro
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Use A123 LiFePO4 cells. They are what DeWalt used (until they switched to Samsung). They can supply over 200 amps! They are nominally 3.2V per cell and have a VERY flat discharge curve. They are also MUCH safer and less temperamental than LiCo cells.

willie
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rufes1 wrote:
yeh, thats what I am worried about… because the original cells are only 1300mah I thought it couldn’t draw too much current, but like you said its that initial inrush that will make the protection circuit kick in…

Arrgghh too hard!

+1 on what the guys say

Cell capacity (1300mah) and cell discharge rate are two different things.