electronics Q - battery recharging circuit design

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electronics Q - battery recharging circuit design

Postby Andrew Davie » Thu Jul 03, 2008 1:10 am

The submersible I'm working on at work carries 6 lithium batteries, connected in parallel. Each is roughly 24V. They are in a sealed hull, with an egress port which can carry wires to the outside world. I'd like to recharge the batteries in situ - that is, without having to unseal the unit, remove the batteries, recharge, seal the unit back up, etc. So the ideal would be to just plug in a recharger and have it recharge the batteries through the wires coming from the port.

The problem is that the batteries must be charged independantly. So that parallel connection of all batteries must first be broken, each battery charged independantly, and then the parallel connection restored. If that makes sense. In short, I'd like to have a circuit which would cycle through all the batteries, charge each, then restore the original configuration.

Now I may be coming at this from the wrong angle. Perhaps it's a simple thing to charge the batteries when they're connected together, but I believe we've been advised this is not a good thing to do.

Another issue might be venting of gases; I haven't checked, yet, to see if lithium polymer batteries are safe to recharge in a sealed environment.

But in any case I thought I'd throw this problem up and see if anyone has some lateral suggestions, perhaps something as simple as whacking on a diode in the appropriate place, and a dab of glue...

Cheers
A
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Re: electronics Q - battery recharging circuit design

Postby Steve Anderson » Thu Jul 03, 2008 12:21 pm

Andrew Davie wrote:The problem is that the batteries must be charged independantly. So that parallel connection of all batteries must first be broken, each battery charged independantly, and then the parallel connection restored. Cheers A


Andrew, you're quite correct in that the batteries must be charged independently, however I must admit that I raised my eyebrows when you said you were going to use them in parallel when discharging.

Now I'm not sure if this is 'legal' with Lithium-Ion batteries but for most other types it's a no-no. Each battery will have a slightly different terminal voltage but with heavy cables between them could lead to some high currents, self-discharge and the risk of fire.

I suggest you contact the manufacturer of the batteries and see if this arrangement is recommended or not.

There is a way around this by putting a diode in the feed from each battery then doing the parallel connection. It depends if you can afford the additional voltage drop of one volt or so.

Also look into a higher capacity battery, i.e. one large battery rather than several smaller ones. It's usually more efficient on space and weight.

Something else worth thinking about is that upon sealing the vessel up its hull is purged of air and replaced with Nitrogen or Carbon Dioxide, with no Oxygen any failure which could have resulted in a fire will result in less damage. I am assuming it's unmanned.

Steve A.
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Postby Klaas Robers » Sun Jul 06, 2008 1:12 am

I have a completely different feeling about this.

Lithium batteries have, contrary to most other type of batteries, an open voltage that is a measure of the batteries charge. When empty about 3,2 volt, when charged 3,8 volt. When measuring the voltage you read the charge still in the battery. And yes, this voltage is somewhat temperature dependent, but if the cells have the same temperature this is not a problem.

For discharging you may cannect cells in parallel, as the cell with the highest voltage discharges most.

For charging a cell the voltage should never rise above 3,8 volt. This applies also for cells connected in parallel. The voltage for each cell is the same.

This does NOT apply to cells in series. Never connect Lithium cells in series. The cell that is first discharged (V = 3,2) will be further discharged by the other cells and will be ruined. In Laptops Lithium cells are connected in series, but inside the battery pack there is a protection circuit for each individual cell. As soon as the voltage of one cell reaches the 3,2 volt, the whole battery will switch itself off. More or less the same happens while charging, no cell will be charged to a higher voltage than 3,8 volt.

So charging your cells in parallel is easy: connect them to a powersupply that has a maximum voltage of 3,8 volt and a max current of the max charging current for one cell. Monitor the current and as soon as this current drops down the battery is full.

If someone knows that the max. and/or min. voltage is slightly different, possible. I wrote this purely out of my mind.

Contrary to Lithium, NiCd and NiMHy cells have a voltage that is almost independant from the charge. It has a negative temperature coefficient, a hot cell has a lower voltage. These cells should never be connected in parallel. One cell can charge an other one, which is not desirable as it costs capacity. Also connecting in series is dangerous, although it is done very frequently.
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Postby Steve Anderson » Mon Jul 07, 2008 3:38 pm

There does seem some confusion regarding this, here's a starting link, the references at the end being useful:-

http://en.wikipedia.org/wiki/Lithium_Ion

also:-

http://batteryuniversity.com/partone-12.htm

http://www.batteryuniversity.com/partone-5.htm

Steve A.

When I refer to a battery in this context I mean a single assembly (usually sealed) comprising two or more cells therefore producing some multiple of 3.2V or thereabouts. Whereas disposable single-cell items (the common 1.5V AA for example) are often referred to as 'batteries'.
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