BMS Basic understanding

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suzukishoes
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Joined: Sun Jun 20, 2010 12:31 pm

BMS Basic understanding

Postby suzukishoes » Mon Apr 18, 2011 11:21 am

I couldn't figure out "How" a BMS works but read this

The BMS will burn up excess current to the cells which are fully charged, allowing the remaining cells to get topped off. Eventually, this results in a perfectly top balanced pack.


So that figures! but can someone give me an idiots guide to a question regarding my G-Wiz (leads) it charges & when fully charged it runs! occasionally it equalises.

Whats the difference between charging & equalising?

If I converted to lithium do the same principles apply? as I know a BMS is an absolute must with them!

Lee

bobc
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Location: Knutsford

Re: BMS Basic understanding

Postby bobc » Tue Apr 19, 2011 12:27 pm

Charging is filling the battery with electricity - equalising is making sure each of the cells which make up the battery is at the same state of charge. I don't know what batteries are in the Gwiz but if they're normal lead acid wet cells, they come in blocks of 6 cells (giving a 12V battery) and you can't get at the intermediate connections. So for such cells it's fairly normal to equalise by slightly overcharging: the non-full cells keep charging until they're full while the full ones gas a bit (electrolyse some of the water in there, giving off H2 & o2). If you don't equalise you get in a vicous cycle where a slightly less full cell is bound to get a higher depth of discharge, which degrades worse than the other cells it until it effectively dies. Then you throw away a whole battery because 1/6 of it is broken....
The standard 14.4V charging regime in a car is actually slightly overcharging & thus performing the equalisation whenever the engine is running. I expect that the car charger normally charges up to less volts than this (say 13.9?) but an equalisation charge is optional which puts a bit more volts on the battery (say 14.4) and you'd use this "equalisation" option every 10 cycles or so.
Stand by to hear from others that I'm wrong on all counts ;^) - I know nothing of the Gwiz, I've just got lead acid experience from elsewhere...

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hohisilver
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Re: BMS Basic understanding

Postby hohisilver » Tue Apr 19, 2011 1:04 pm

If I converted to lithium do the same principles apply? as I know a BMS is an absolute must with them!


By contrast, lithium cells will not tolerate overcharge in the same way as lead-acid, so the BMS equalises them by the aforementioned "burning off" of current (although there are other methods which are not so wasteful) to individual cells that are fully charged, thus (hopefully!) ensuring that fully charged cells receive no more charge whilst those that haven't reached that state continue to charge.
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

suzukishoes
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Joined: Sun Jun 20, 2010 12:31 pm

Re: BMS Basic understanding

Postby suzukishoes » Wed Apr 20, 2011 8:57 am

Thanks bobc & Ian. Just for information the G-Wiz has 8 x 6volts in series giving 48v total.

So with lithium if you have the right BMS there's no extra regime just plug in?

Lee


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