7p10s Lawn Mower Battery Rebuild

hey guys this is Frank I just want to do
a follow-up on the battery that I built for my lawnmower it works but not as
well as I thought it should there have been some issues with charging and
balancing and the BMS that I have right now just isn’t working so I’m going to
kind of show you the battery kind of walk through some of the things that I
think is the issue and I’m gonna just start doing some testing on it to learn
from the process and see if there’s something I can do to make the battery
better I have been able to mow my lawn a couple times with it and the first time
I was successful went all the way through but then when I went to look at
the battery it was like dead one of the one of the cell banks was like really
low so I quickly charged it up and everything worked fine it didn’t seem to
have damaged that cell bank but it just wasn’t balanced correctly even though I
had charged it correctly at the beginning and I think it has to do with
the BMS or it may be a bad cell or two in that one bank so let’s take a look at
the battery itself okay so this is the BMS itself here and
that I have and I I don’t know for sure that is actually bad but it just doesn’t
seem to be functioning correctly if I look at see if I can turn on my charging
unit so you can see this is now fully charged I think that’s a good angle for
you to see these volts and if we look at each of these these cells let me take
that over to one more so you can see the actual so this is 4.0 706 and then if I
go over to this cell here this cell set 4.0 708 I mean it’s close it’s not exact
where I want it it’s a new years 4.7 so that’s that’s good turn this here so I
can see then this cell bank here 4.6 4.7 so that’s close and this one here 4.7
again that’s good io philia and we have this one here is 4.6 so we’re all
getting right in there 4.6 4.7 it’s looking really good you know 4.6 there this cell back here 4.6 again and then
this one here is the one I’ve been having grief with it’s not a 396 I mean
that’s I mean it’s only a few point volts below what everything else is but
it’s really it they should all be right spot-on
and that’s 1.4 4.1 7 which is technically a 4.2 so that’s like max
voltage that you’d want to go on the cells so I have this cell bank here that
is not whatever reason the charging just is not consistent and I’ve used my
balanced charger and actually have balanced each of these I run through
each of these cells and charge them independently and get
them all perfect I can go out and I can mow more works great last for the entire
yard come back in and I can see that the battery is drained but this one cell set
just keeps getting hit when I recharge it it just doesn’t seem to be right I’m
thinking it might be the BMS or it could be a cell in this set the way I would
I’m going to do the BMS first because that’s kind of a simple piece the next
one to check each of the cells is I would disconnect each of these solder
joints on the positive side over here and then check each cell independently
because right now if I check any of these if I go down through any of them
they’re all going to show basically what the bank has in it if I have one bad
cell that one cell will still show you know it’ll appear like it’s good because
the rest of them are all carrying the weight and so I just need to really
narrow that down to figure out why that one cell bank is not charging correctly
and why this one is overcharging to me that would indicate that it’s the BMS
that it’s not doing a balance function and it’s not functioning the way I think
it should other than that mowing went fine I was able to mow
without any issues other than that first day when I I don’t know what happened it
literally took me all day to mow my lawn because I was charging this battery and
charging my let acid batteries and switching them out and it just took
forever and it was crazy so I’m gonna just take a minute here and we plug in
my soldering iron I’ll let this get hot and we will I’m gonna go through and
remove these leads and replace this BMS so here we go we’ll get to work on this
all right so I’ve gone through and worked out my leads I had to go through
and redo all my leads to go from this old BMS which I think is my issue but
I’m not positive this is just gonna be the first step in this whole process of
checking to see what the issue is with that one cell bank or actually I had a
couple cell banks once overcharging ones under charging so I’m switching vmss and
I went through and started the soldering process
and I was filming it and my battery on my camera died and so I wasn’t able to
show you the fact that I crossed touched the lead and had a big explosion it was
light show but camera didn’t catch it so you guys don’t get to see it but I
enjoyed it it was fun okay so what we have here is I need to go through before
I go plugging this my new leads into the BMS need to validate that they’re
soldered correctly so that I don’t hurt the BMS and part of that process I’m
just gonna wedge this negative lead in here so I can check each of the cells so
how this BMS or BMS is work is they have these the lead here has multiple pins
and starting with the black wire over here on this side going across I’ll call
that pin one two three four there’s ten of them toward this pack and the way it
works is is that black wire pin one goes to the negative and then pen two goes to
positive and so if I touch the positive over here it’s four volts and then if I
go to pin three which is over on this side it goes to eight volts and so on
and so forth so what I need to do though is validate that they are correct on
here before I go plugging into the BMS and the way I do that is I have a wire
here it is I need to just use a pin because these pins are so small in here
so I’m gonna start by going into pin two here and then I’ll just touch this and
it shows that so pin one to pin 2 is 4 volts and that is correct and then if I
go to pin 3 it should go to 8 volts and it does and then we go to pen 4 and
that’s 12 volts and I just need to walk through each of these we should have
increments of about 4 volts each 16 and there’s 20 tiny tiny pins 24 so what 28
yeah 28 for looks good and 32 next one is 36 and Lawson should
be 40 roughly 40 yep forty point seven okay so my leads are all done correctly
and it looks like everything’s in order there so now I can actually plug this in
to the BMS and I will heat up some hot glue and mount this down so it’s a
permanent fixture and I’ll do some cleanup of the the cabling so it lays a
little neater but other than that the other thing I wanted to show you is on
this BMS it was a little different than the previous one this one has different
connections there’s a b- which goes to the battery it’s a it connects into the
the battery itself of negative pole in the battery then there’s AC negative
which is the negative part of the charging side so this is now my charging
cable and it will do the charging so the charge current will flow through here
and then into the battery actually it flows in the other direction but that’s
the current piece there then there is p- which is going to be the load it’s going
to be the item i’ll be running this will be plugging into the lawnmower itself
and I put different ends on here so I didn’t confuse them one of them is
charge and the other one is load or going to the lawnmower itself so I had
to do that the other the other BMS actually shared it didn’t have a charge
lead it just had a P and so the P shares that responsibility of charge and this
one here runs a little separate I think it because this one here I’m hoping if
any of you have experience with this and know the answer to it I think it’s
because this one actually has true balance properties in it and it does
battery balancing and that’s why it has a separate lead is so that it can
actually move the charging appropriately so
I’ll do some more testing on it and then I’ll report back I don’t know that I’m
gonna spend a ton more time on a lot more stuff I have other projects I want
to work on and this one here was one I just needed because I needed a lot more
and I just thought it’d be fun so now we get to move back on to the portable
power pack and moving into solar and some other things that and the power
walk and I call these batteries I got to start building my power wall and we’ll
just have to keep doing updates on that and see how that goes so it’s gonna be
kind of fun I’m actually looking forward to that all right with that I’m gonna
call it I just appreciate you time

9 thoughts on “7p10s Lawn Mower Battery Rebuild

  1. The charge input is not all that special, all cheaper BMS boards use top end single cell discharge balancing, the cell gets to 4.2 and it switches a resistor across the cell to discharge it. It probably also turns off the charge input ( or at least slow it down with PWM) you should test it with a clamp meter on the balance leads, I haven’t seen many videos of BMS behavior in action

  2. If mowing your whole yard completely discharges your pack then the pack is not big enough. Is there room for a second pack connected in parallel on top of that one. Adding a second pack should half the imbalance as well as reduce draw and double your run time. For whatever reason your pack is discharging unevenly. Probably different types of batteries with different internal resistances. If you are going to charge with a balancing charger then why do you need a BMS? Your pack will balance every time you charge it which should be every time you mow. My experience with off the shelf BMS boards is that they don't work on large packs because they have a limited discharge ability and the rate of charge on a large pack exceeds their ability discharge fast enough. They do nothing when discharging! they only discharge cells that reach 4.2 volts to keep them from overcharging. A BMS that was able to balance while discharging would have to have both battery leads and leads for your load separate from each other on the BMS. Something like this would probably need to be custom made or maybe something like the opensource BMS project I saw would be configurable enough.

  3. we have a very similar situation with the mowers! As for the BMS i have heard many situations where people have a BMS that cuts power but does not do true balance. The one you have now may actually balance however it only balances on the top if the charge, from what i hear. So, that means you have to charge it, let it get to full then let it sit plugged in and it balances the cell banks at a very low amperage. So the balance feature may take quite some time. Get a hold of the BMS seller asking about the balance feature. If his name is micah (look at shipping label) then try heading to ebikeschool.com or the youtube channel. Lots of BMS info there. good luck with it!

  4. best way and i have this problem with all 4 of my 4S bms's capacity check the one bank and compare it to the others with out the bms connected and see what the difference is. If the diffrence is minimal then yes its the bms causing the problem. You can try another one or add a parallel cell to that one bank to get it more balanced with the others through the bms its been my experience that only one bank of the S series out of the 4 with mine that will do this ya just gotta find it and add a parallel cell to offset the difference. I went nuts finding this problem when i seen it come up much like yours one cell row runs down faster while the others still have 3.2 to 3.5 volts and the pack shuts down because of that one row. You put alot of effort in the pack so verify cell bank capacity to confirm the bms is acting odd. Hope this helps.

  5. Hello Frank, I am hoping you could help me with this question. I made 3×24 18650 battery packs wired them in series to get 60ah @ 12v and added a 100amp BMS. Can I make 3 exactly the same packs and wire them in Parallel to acquire 120ahs? Thank you for your feed back. Have a nice day.

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