36v charger TEMPORARY on 48v system

[docrocket]

So I finally finished assembling my 13S3P SPIM08 battery pack monstrosity. This is a straight-up experimental build and I suspect I'm going to have reconnect everything after a test ride or three.

Unfortunately the 48v charger I bought with the rest of the bucket of parts was DOA. So I have a constructed battery pack with basically a full charge (52.6V) but no charger. Ordered a new one, it'll be here Monday night.

I'm going to be running this battery with an old no-name 36v controller - it's confirmed as having the voltage capacity to run 48v. The battery has a 13S BMS installed.

I'd like to take it for a range test. I also don't want to have to keep swapping in and out the 36v menagerie I've been powering this bike with. I know that charging a 36v on a 48v charger is Very Bad Mojo. But it seems like it should be OK to do the the other way around - it'll just give them a partial charge to around 3.5 volts/cell. Which is fine; the SPIM08s can go to to 2.5 volts before the cutoff.

I also have a SkyRC B6ACv2 hobby charger (which is how the cells got charged to begin with). Can I use that to somehow charge the whole pack or do I have to tediously disassemble-reassemble?

Main question: is it safe/non-damaging to charge my 48v pack at 36v for a few days while we wait for a proper charger? Thanks.

 

[goatman]

if you open up the 36v charger you might see a blue pot with a brass screw on the board to turn the voltage up. if you do open it up and there is one or three pots, take a picture and upload it here so someone can help you

 

[amberwolf]

First, if you want to do a range test, it is *completely pointless* to use any charger voltage lower than the one you will use with it "for real".

If you use a lower voltage charger, you will not get the range you would from the full voltage, so you can't test the range of the pack.

Some more discouraging calculations below:

Main question: is it safe/non-damaging to charge my 48v pack at 36v for a few days while we wait for a proper charger?

Damaging? No. But it wont' really be charging it, because a 48v pack is pretty much empty at the full charge voltage of a 36v pack.

If your BMS is programmable to change the cell-level LVC, then you can run the cells down to really dead (which certainly can damage them, or at least wear them out faster), in which case the charger will at least recharge them to just their normally-desired shutoff point.

I doubt you'll have much capacity, maybe a few percent of the total at most, and that only if the cells are all well-matched and in good conditions. You'd have to check the spec sheet for the cell in question to see how much capacity that cell is supposed to have when factory new condition, from it's minimum voltage to the voltage they'll end up at with a 36v charger, which is about 42v / 13s = 3.2v per cell.

Let's say you get 5% (being generous) capacity within that range, and let's say your cells are perfect brand new perfectly matched 8Ah cells. 3x 8Ah is 24Ah; 0.05 x 24Ah is 1.2Ah.

So if that's approximately correct, you'll get around an amp hour of capacity total, or something around 50wh-ish.

If you are using this for a typical ebike build, and riding at typical bike speeds, you'll probably use around 20Wh/mile. So you might get three miles out of it, if it's a perfect pack of perfect cells.

 

[amberwolf]

I also have a SkyRC B6ACv2 hobby charger (which is how the cells got charged to begin with). Can I use that to somehow charge the whole pack or do I have to tediously disassemble-reassemble?

Why would you have to disassemble it? You simply connect it's main and balance wires to one "set" of cells at a time within the pack, however many it can charge at a time. This would bypass the BMS, so it won't be able to shut off charge or discharge, but if it has monitoring ability you can use it to keep an eye on the cells as they're charged, and it could still do it's balancing thing if it's a balancing type.

If it's a 6-cell charger, then you could charge in three steps.

First step you connect the charger negative to the main battery negative, then connect each balance wire of teh charger to the appropriate cell positives, for a total of the most negative six cells in the pack. Then the charger postiive to the positive tab of the sixth cell. (don't connect this wire to the balance wires as they are probably not thick enough to handle the current.

Once that set is charged, then you can disconnect all those wires, then move them to the next set of six cells. The negative would go where the positive just was, then each other wire up from there.

Then the last cell group is charged as just 1s , so you don't need the balance leads and can just ocnnect the charger + and - to the cell tabs.

 

[docrocket]

By range test I mean ride it into the dirt on the full charge that it has, then be able to charge it back up some so that it would be usable again while I wait for the other charger.

But if I understand you correctly, I can charge individual cells in my pack even while they're connected to the rest of the battery? I guess I always thought the cells would self-discharge, perhaps disastrously, if I did something like this.

My hobby charger has balancing capability but I don't have balance wires. So my pack consists of 13 sets of 3, and you're saying I could charge it one set at a time, just go down the line for each set of 3?

Thank you for your help and patience with my glacial learning process.

 

[docrocket]

if you open up the 36v charger you might see a blue pot with a brass iî

This is the closest thing I find:

https://imgur.com/gallery/Ell1FvL

 

[goatman]

when you upload that picture to imgur you can resize it for message boards and bring the picture into this thread. ya those are pots. normally the one that is by the chord that leaves the charger is the voltage and the other ones are for watts. if you increase the volts coming out to 54.6v for 13s you need to turn down the watts so you don't blow the charger. my guess would be the pot by itself is the volts. theres a thread here about doing this, ill go find it

https://endless-sphere.com/forums/viewtopic.php?f=14&t=104061&p=1520456&hilit=adjust+charger+pot#p1520068

 

[docrocket]

when you upload that picture to imgur you can resize it for message boards and bring the picture into this thread. ya those are pots. normally the one that is by the chord that leaves the charger is the voltage and the other ones are for watts. if you increase the volts coming out to 54.6v for 13s you need to turn down the watts so you don't blow the charger. my guess would be the pot by itself is the volts. theres a thread here about doing this, ill go find it

Hey thank you sir, extremely helpful. I shall read up and (maybe) try it.

 

[amberwolf]

But if I understand you correctly, I can charge individual cells in my pack even while they're connected to the rest of the battery? I guess I always thought the cells would self-discharge, perhaps disastrously, if I did something like this.

If they self discharge then there is a problem with the cells and they will need to be replaced.

I don't see how a charger could do that in this process.

My hobby charger has balancing capability but I don't have balance wires. So my pack consists of 13 sets of 3, and you're saying I could charge it one set at a time, just go down the line for each set of 3?

No, what I said in the steps I outlined, is charging in a set of six groups, and a set of six groups, and a set of one group.

But yes, you could instead charge just one parallel group of three at a time--it will just take a lot longer.


As a note, if you don't have balancing wires, how do you expect your BMS to protect them?

Or are you referring only to the RC charger?

 

[kdog]

In my experience the trimpots are for fine tuning and won't give you more than a few volts either wsy. You won't get more than about 45volts out of it before you're outta turns. But if youre lucky, absolutely turn down current output to maintain same output wattage, otherwise she blow quick (just a minute or two)

 

[LeftieBiker]

By range test I mean ride it into the dirt on the full charge that it has, then be able to charge it back up some so that it would be usable again while I wait for the other charger.

The first cycle of a pack usually is not representative of what the capacity will be after a few discharge/charge cycles. What you will get is, very roughly, 60-80% of what the range will be when the pack is 'broken in'.