# Simple lead acid BMS



## few2many (Jun 23, 2009)

Wow, no one has replied to this?
I like the idea and dont think it'll be too hard to adapt.
With all the solar and wind power stuff I've been researching, cant believe i didnt make the connection!


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## jackbauer (Jan 12, 2008)

I have a few on order from ebay and will be giving it a try when i get my charger running.


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## SimonRafferty (Apr 13, 2009)

It says it will cope with 10A load - this can obviously be increased.

I presume you have had some correspondence with the seller as the listing does not give much away? Are you able to reproduce it here?

The 14.4v 'regulation point' is lower than I would like - but again this might be adaptable.

Looks like a good basis though.

I've been having some thoughts on a Lead Acid BMS. Dimitri's MiniBMS is obviously for Li cells - and indeed he says he's not interested in Pb as they are too "Last Century". Still they see a lot of use in EV's and maybe we need a similar open source Pb BMS thread?

Since the DIY Charger now has a BMS input - it makes sense to incorporate an output into a BMS design.

What I was considering was simply a Zener diode (or several in series to give greater voltage control) driving a PNP darlington, driving a DPST relay. The relay switches in a shunt resistor or light bulb as well as providing an isolated pair of contacts to feed in to the charger.

The charger will gradually reduce the voltage when it sees any of the batteries shunting so the shunt resistor need only be low wattage. 

Si


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## jackbauer (Jan 12, 2008)

I haven't had any contact with the seller it was just an example. I'll be looking at it more detail when the charger is finished. My basic idea was to power a relay from the output and wire them in series back to your charger. It just struck me as an off the shelf solution. A 5w zener and a mini relay or opto would suffice to send a signal to the charger.


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## dtbaker (Jan 5, 2008)

I don't think you NEED a BMS to shunt balance w/ FLA. You want to over-volt them to gas, then back down the trickle to top them off.


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## Jimdear2 (Oct 12, 2008)

Hi All,

I have a question about BMS setups (at least the types that I can understand) that has really bothered me. Please understand am NOT very good with electronics.


In all of the BMS that I have looked at and understand, there seems to be three major functions:
Not allow the individual cell voltage to climb above a certain voltage (fancy systems use temperature corrections) when charging
Not allow the cell voltage to drop below a certain voltage (fancy systems use temperature corrections) when drawing power from a battery.
Make the end user aware and in some cases perform a function when one of these situations occures.
Here are my questions and points.


Points

When a cell reaches a certain voltage most of the systems seem to shunt the charging power to a resistor that wastes this power (I empisize WASTES) as heat (sometime causing a fire).
When a cell reaches the low voltage point the BMS does any of a number of things, like sounding warning buzzers, flashing lights and up to, I beleive in one case, cutting power to the motor. 
Questions

Are there not electronic switching devices that could switch the cell out of the circuit, bypassing it when a low or high voltage event occurs. Something that could still pass the necessary current from the remainder of the battery when in normal operating or bypass modes.
This way when the high voltage event happened the fully charged cell would be bypassed and isolated, the charging current would be passed around that battery and used to charge the remaining cells with the charger reducing output current as needed instead of wasting it as heat. When charging was complete the BMS unit would switch all cells back into the battery circuit.
When the low voltage event occured the faulty cell would be switched out of the circuit and bypassed while the operator would be notified. the BMS would identify the battery at fault in any number of ways depending on the complexity of the hardware and software.
Probably what I describe is way too complicated or expensive, that's where you EEs come in. 

I do have to admit though that when I hear people talking about saving energy and then talk about wasting it as heat each time they recharge, it confuses me a bit.


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## few2many (Jun 23, 2009)

As I understand it, for solar and wind charging, you want a shunt because the solar/wind may still be generating after the full charge. On an EV, wouldnt you want the bms to simply disconnect the fully charged cell, so the other cells can use the power that would be wasted as heat? When all the cells are disconnected, charge is complete.
Maybe have a shunt, resistor or light bulb set up on a timer? A timed trickle charge.


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## dtbaker (Jan 5, 2008)

I don't think switching a cell (or battery) out of the pack is an option unless you had a contactor capable of handling full pack voltage on each cell.... which would be hugely expensive and take a ton of space....


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## SimonRafferty (Apr 13, 2009)

dtbaker said:


> I don't think you NEED a BMS to shunt balance w/ FLA. You want to over-volt them to gas, then back down the trickle to top them off.


I think you are confusing "Want to" with "Can get away with".

If I charge my fork-lift with the supplied charger, during the equalization phase, it gasses, and bubbles for 4 hours. Then I have to add about 3 pints of water (in total) to the cells.

If I charge it with my managed charger, it does not fizz, bubble or gas and I do not have to add any water at all. However the cells are equalized and I get the same run time.

A lot of people are running AGM or sealed batteries - so allowing them to gas is really bad as you cannot fill them up. In this case, they do NEED managing and ideally BMS - No?

Si


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## few2many (Jun 23, 2009)

Sorry, my mistake for not clearifying. Absoluely, if you try to disconnect one bat at full pack voltage, switching would be an issue. Plus, its like the old christmas lights, one light out and the whole circuit opens. 
What I was referring to was charging batts individually, with they're own bms that opens or timed floats when at full.


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## dtbaker (Jan 5, 2008)

SimonRafferty said:


> I think you are confusing "Want to" with "Can get away with".
> 
> If I charge my fork-lift with the supplied charger, during the equalization phase, it gasses, and bubbles for 4 hours. Then I have to add about 3 pints of water (in total) to the cells.
> 
> ...


AGMs do have a different suggested charge curve which equalizes without gassing..... I have never heard of shunts or BMS required for lead-acid charger, just different charge curves.


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