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Step down from 42v to 36v
#1
I managed to get hold of 2 Lawnmower batteries, each with 10s 2p cells producing 21v odd.

I soldered them in series to get 42V, the cells are very good quality, although I can't find the brand and see no reason to take them apart when they are so well made.

My problem is my drill found the 41V a bit too much so I am going to get a step down device.

My question is How many amps is the 42v pack using?

I can't take the cells apart and there is nothing on them anyway as far as I can see, I imagine they are 2200 or maybe 2400 per cell, but I have no means to test that.

I tried putting a meter I got from eBay which was supposed to measure up to 60v but it cooked it! 

I probably did not get the wiring right, problem with drill projects is the same connectors feed the load as well as the charge, so I plan to put a DC connector in which will work with my new 42v charger.

I got a great meter for showing battery capacity, but it only measure voltage and bars of energy left

https://www.ebay.co.uk/itm/263654026311

I am struggling to find step down that does the right voltage and fits in the space I have available.

I know I could just exclude a pair of cells but it is all soldered in so nicely including nice terminals.

I would appreciate some guidance on Amps and if you know a small device that would be helpful.
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#2
What type of cells? There aint any lawnmover batteries our There doing 2.1v per cell.....



Images please. I think you missed something. Or you got some weird chemistry
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#3
Your drill is probably pulling close to 10A-15A at startup. It'd be hard to get a good buck converter in a low cost package that can handle that high of amps continuously. For a drill, hammer, impact, or circular saw, the amps are sustained very high.
18650 likes this post
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#4
(03-15-2020, 10:23 PM)18650 Wrote: I managed to get hold of 2 Lawnmower batteries, each with 10s 2p cells producing 21v odd.

I soldered them in series to get 42V, the cells are very good quality, although I can't find the brand and see no reason to take them apart when they are so well made.

My problem is my drill found the 41V a bit too much so I am going to get a step down device.

My question is How many amps is the 42v pack using?
if the lawnmower batterys are 10s2p and you soldered them together in series you now have a 72v battery.
10s is a 36volt nominal setup. sounds like the batteries were way dicharged unsolder them and charge the batteries .
your drill is most likely a 18v or a 24v drill 5s/6s

later
Floyd
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#5
====================
if the lawnmower batterys are 10s2p and you soldered them together in series you now have a 72v battery.

10s is a 36volt nominal setup. sounds like the batteries were way dicharged unsolder them and charge the batteries .

your drill is most likely a 18v or a 24v drill 5s/6s

later

Floyd
=========================

Sorry if I used the wrong description, is it 5s2p then?

If you look at the layout there are 10 cells in each pack, every two are in parallel and then in series, it is as follows:

1-1 4.2v
2-2 4.2v
3-3 4.2v
4-4 4.2v
5-5 4.2v

So 21v each battery and 2 x 21v = 42v

My Drill is 36V it is an industrial strength drill which is why I want to keep it.

It previously had 30 x 1.2v Ni-CD cells, 30 x 1.2 =36, the other clue is all the 36v labels on the drill and the charger.

I have not had to solder them indivividually, I was able to take the packs out of the Lawnmower and I just soldered them together in series
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#6
Then its 5s2p Wink

You can step down but even better dont Charge Them ti 4.2v. IF you stop att 4v you instead get 40v. And 10s is 36v nominal so should be fine
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#7
(03-16-2020, 12:13 PM)daromer Wrote: Then its 5s2p Wink

You can step down but even better dont Charge Them ti 4.2v. IF you stop att 4v you instead get 40v. And 10s is 36v nominal so should be fine

Ironically I bought a charger that can do 42v

I am not sure what it is in the drill that is unhappy, but it is around the connectors and it started smoking around the connector so I figured best to do something!

(03-15-2020, 10:57 PM)daromer Wrote: What type of cells? There aint any lawnmover batteries our There doing 2.1v per cell.....



Images please. I think you missed something. Or you got some weird chemistry

So I guess this confirms they are 5amps each and so 10amps for the two


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#8
I can't find any step down devices small enough.

So it seems I have two choices.

1. Open up the drill and find out what is smoking, if it is wiring then replace it, but otherwise seek to lower voltage.

2. Remove two of the cells from one of the two battery packs, this would make it 9 x 4.2v so 37.8v

My only question is how would this affect the BMS which is a 10 wire one?

Do I just leave wire 10 out of it?
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#9
what kind of connectors and wiring are you using? maybe they are too small for the amps. If the connectors are too small, they will start to heatup when you pass amps through them.

If you remove cells then you will need a new bms. If you leave one wire disconnected, the bms won't work. The powertool bms are very picky, they have to see the right amount of batteries and correct voltage for them to work.
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#10
It more sounds like you wired it backwards or something. It should not burn due to this unless the old pack was 3.2v cells
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