NiMH powertool upgrade

Nemo

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Joined
Apr 26, 2019
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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/Heaterchecks. 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.
 
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 :)
 
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.
 
jdeadman said:
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?
 
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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jdeadman said:
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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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-Batt...ywords=5s+BMS&qid=1556762420&s=gateway&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.
 
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.
 
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
 
Geek said:
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?


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

I"m still getting used to all the terminology. I know its a balance charger if it has wires going to each series connection right? So the unit I posted above would be considered a balance charger?
 
Multiple protective functions including overcharge protection, overdischarge protection and short circuit protection

If it provides these protective functions on each cell in series I think you are right it would be self balancing

Would like to get feedback on this board. I have been looking for something similar but hoping for half that cost.
 
Go for a BMS with balancer, and a relatively low-current PSU. Or grab two PSUs, a big one to top your batteries quickly and a small one (max 200mA) to allow enough time for the balancer to balance the battery. You may omit the BMS completely ONLY if you use a balance charger for charging, but you're running on extra danger of damaging a cell that way due to over discharge, however it won't catch on fire, fire happens when charging the battery.
 
Okay, now I'm confused. I've been thinking of BMS and Balance Chargers as being the same thing, but it sound like they are different or is it that the BMS is doing the balancing, but the power for the BMS has to come from a charger with a little more brains than, say, a laptop power supply.

So in the case of this board, I thought I could just plug in a laptop power supply to the provided power jack and this circuit would manage the rest. It does claim to have overcharge protection. IF that's not the case, how does a person determine what is needed?
https://www.amazon.com/5Series-Batt...ywords=5s+BMS&qid=1556762420&s=gateway&sr=8-3

If this is the case it would represent a pretty good deal overall as I could salvage the PSU and then the battery would cost $11 plus batteries and materials.


Just found a good breakdown here:

In this case the circuit he uses (4:34) doesn't have enough wires to be a balance circuit, so he adds an additional harness to use his IMAX B6 for charging/balancing. Otherwise if he put it in the regular tool charger (if it worked at all) it would charge blind and could unbalance.

So this makes me think the circuit I've linked IS a balancing BMS, and as Thanar suggests I could use it and a low current PSU. So here I go, placing the order. I'll give it a shot and let people know how it goes. This will be my first battery build of any type...
 
Follow-up! I've received the circuit board.


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- There was no manual or data sheet of any type included but the board is well marked enough to figure out what goes where
- Model number is XL-MH9959.
- MOSFETs are SM3116NA which are apparently rated for 30V and 60A data sheet (http://www.sinopowersemi.com/temp/SM3116NAU_datasheet.pdf)
- Main IC is BM3451, a 3/4/5 charging protection and balancing chip data sheet (https://datasheetspdf.com/pdf/1098180/BYDMicroelectronics/BM3451/1)
- Wire for main battery connections appears to be 16AWG which is sufficient, but all the wires are very short and may need to be replaced
- The battery clips showed no hint of copper, unlike a Makita charging board I was comparing it to, soI thought maybe I'd got nickel plated steel, but after filing one of the clips there was copper in there.

So it seems legit and likely it will be able to handle the 20A it was marketed as rated for. If anybody spots anything fishy or want more part info let me know.

Looking up the model number there is a blog from someone in Hungary reviewing the board for use in a DIY Drill Battery Build - update 2018-4-25(https://electrodiy.blog.hu/2018/03/31/akkumulatoros_csavarbehajtok_akkupakk_felujitasa_hazilag). Google Translate does a decent job and it sounds like he he was happy with it after a few modification to work with his drill and charger.

I plan to be making some modifications as well. I'm hoping to set this up to be chargedby a 20V lenovo laptop charger. Building the actual battery to go with it is going to be a bigger challenge. I have the cells from other power tool batteries, but tossed everything else before I knew that tools often use thicker nickel or nickel plated copper for their connections. I could have reused the nickel plates if I was more careful tearing the pack apart. Going the straight nickel route (if I can find any, still looking) at 20A peak with 2p packwill require 2 layers at the series connections...probably going to end up looking ugly, oh well, function first.
 
Hey nice. Yea that's the pack. Mine has been working fine but it's not pretty. the cells are standing upright in my holder. I'll have a look at yours.
 
nice. i'm using mine in a skill saw and wanted a bit more run time so i have 5s3p of the brown 3000mah cells
 
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