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NiMH powertool upgrade
#1
To get my feet wet before moving on to bigger pack builds I thought I would try my hand at upgrading a few NiMH powertool batteries to Li-ion.  I've salvaged cells from powertools packs, and done the standard capacity/SD/Heater checks.  I'm aiming for a 2p5s pack to replace the NiMH on a dewalt 18V drill, but I'm stumbling on the charging/BMS/Balancing portion of the build.  I don't have an existing Li-ion charger for the tool or a compatible BMS from the packs I broke down (because I tossed them before learning I should keep them).  I'm thinking of converting probably 3-4 batteries, but can't rely on getting the same brand tool packs (and therefore BMS) so I don't think investing in a branded charger is the way to go.  After reviewing other posts in to forum it seems to break down to three options:

- BMS with charging management and a dumb power supply
- BMS without charge management and a charger
- No BMS and a charger

I was hoping to get some feedback on the pros/cons of the various approaches with respect to safety, functionality, and cost.
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#2
BMS should be a must for such a setup. Because then you have voltage protection for discharge and you can have a dumb cc/cv psu to charge them. If you skip the BMS you need a balance charger like any RC charger.

The original charger can be binned for something else Smile
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#3
I have one setup for my Cordless Skill saw that was 18V and is now 5s3p lithium Ion. I had to make a new bottom for the case as the cells are longer but with a 3d printer it came out just fine. I do use an RC style charger to balance charge mine but Since I was careful to choose only 1 model of cell and match the capacity exactly (down to the mAh at the rated draw) the charger does very little balancing and I have also charged it off a non Balancing charger from time to time with zero issues. I'll take a couple picks of the finished battery but I dont have one with the new bottom off.
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#4
(05-01-2019, 07:26 PM)jdeadman Wrote: I have one setup for my Cordless Skill saw that was 18V and is now 5s3p lithium Ion.   I had to make a new bottom for the case as the cells are longer but with a 3d printer it came out just fine.   I do use an RC style charger to balance charge mine but Since I was careful to choose only 1 model of cell and match the capacity exactly (down to the mAh at the rated draw) the charger does very little balancing and I have also charged it off a non Balancing charger from time to time with zero issues.   I'll take a couple picks of the finished battery but I dont have one with the new bottom off.

so you went the "no BMS" route in favour of a more expensive RC style charger?  How do you manage discharge protection as Daromer mentions?
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#5
Since my Saw starts so noticeably slow down when the battery is still only half charged I always stop when I notice that. usually leaves the cells @ 3.8V at rest and while cutting I notice it when the cells show 3.6V So it's pretty safe. With the lithium (8Ah vs 2Ah with nimh) the voltage drop seems to start sharpening at that point. but with the added capacity I find I can now use it about 4 times as long

(05-01-2019, 10:25 PM)jdeadman Wrote: Since my Saw starts so noticeably slow down when the battery is still only half charged I always stop when I notice that. usually leaves the cells @ 3.8V at rest and while cutting I notice it when the cells show 3.6V So it's pretty safe. With the lithium (8Ah vs 2Ah with nimh) the voltage drop seems to start sharpening at that point. but with the added capacity I find I can now use it about 4 times as long

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#6
I don't think I could rely on that method with a drill. I think the running amps are much lower for a drill than a skillsaw. I'm wondering about these units: https://www.amazon.com/5Series-Battery-P...way&sr=8-3
it looks like they handle everything and I would only need a dumb PSU for charging. Am I missing something? THis seems like the cheaper and safest route overall, one PSU, $15 BMS per battery pack, though I'm not sure about the 20A rating.
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#7
If I was going to use a Lithium battery without a BMS, I would just put a battery alarm on it. I would also be inclined to use a fuse as well.
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#8
I've not had a problem but If you are worried totally go for a cutoff device or warning beeper. But I don't think a balancer is needed
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#9
(05-02-2019, 02:47 AM)Geek Wrote: If I was going to use a Lithium battery without a BMS, I would just put a battery alarm on it. I would also be inclined to use a fuse as well.

First i've heard of Battery Alarms, is this something you custom build, or are there pre-made modules that can be bought on Amazon/Ali?

(05-02-2019, 04:44 AM)jdeadman Wrote: I've not had a problem but If you are worried totally go for a cutoff device or warning beeper.   But I don't think a balancer is needed

I'm not putting as much effort into matching brand and capacity as you, so a balancer might be applicable for me, though reading the specs on the unit I posted a link to it does not appear to balance? or is that such a standard feature for BMS that nobody even mentions it?
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#10
This type of larm works and you can set alarm voltage on em.


Dishchare without BMS is not that big of a problem. You notice fast when you get to low voltage and as long as you understand that you just stop when you hit that point. Then you just charge it up with a balance charger again.


The balance part for me is cruical. Especially if you run 2nd hand cells. A balance charger does not cost that much so.
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