Battery charging

Discussion in 'General Electronics Chat' started by briangreen, Dec 4, 2008.

1. briangreen Thread Starter New Member

Dec 4, 2008
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Hi everyone, I'm new to the board!

I've come asking a general question about lead acid battery charging. I did a little research on the net and found there's a common 3-stage algorithm to charge lead acid batteries: Bulk charge (constant current), Absorption charge (constant voltage) and Float charge (LOWER constant voltage).
The diagram on the second page of this pdf shows the general algorithm graphically: http://www.manson-power.ru/files/SBC_21101520_2007_eng.pdf

My issue is with the last of these 3 stages. If you put a voltage lower than the battery voltage across its terminals, then surely you don't get an increase in battery charge? If there isn't a diode on the output of a charging circuit I would have thought if anything, that you'd get a large reverse current.

Can someone please explain how this works? I used to think I understood electronics..

2. studiot AAC Fanatic!

Nov 9, 2007
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Well hello briangreen, welcome to AAC.

Can't see much to worry about in your posted spec sheet except poor use of English.

But then 'twer ever so.

I would guess that the voltage levels around 13 volts referred to are trigger levels like the the 9 volt trigger referred to elsewhere in the sheet.

I think you will find that when in 'Float Mode' the charger cuts in, as use of the battery takes its terminal voltage below 13.whatever and out again when the trickle has taken the terminal voltage back above 14.6.

3. SgtWookie Expert

Jul 17, 2007
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In "bulk charge" mode, a constant current is fed to the battery until the voltage reaches ~80% charged state.

In "absorption charge" mode, a constant voltage is placed across the battery until the current falls below a threshold level.

In "float" mode, a much lower voltage is placed across the battery indefinitely. At around 25°C, the float voltage should be around 13.7v for a 6-cell lead-acid battery.

The correct actual charging voltage and float voltage will vary inversely as a function of the battery internal temperature, about -3mV per cell per °C delta from 25°C; for a 6-cell 12.7v battery the delta will therefore be about -18mV/°C. Failure to correct for the internal temperature of the battery will result in decreased service life of the battery.

Last edited: Nov 15, 2010
4. studiot AAC Fanatic!

Nov 9, 2007
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So perhaps Brian is right and elctronics has changed?

Ford used to say that a freshly charged auto battery could reach 14.6 volts.

5. briangreen Thread Starter New Member

Dec 4, 2008
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Hey guys, thanks for the reply! I was forgetting about the internal resistance of the battery! doh!

6. SgtWookie Expert

Jul 17, 2007
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You get different numbers depending upon battery manufacturer and battery type.

Let's say a typical float voltage might be 13.56v with the internal temp @25°C.
The same battery's float voltage @-33°C would be 14.6v, @56°C, 13v.

Schumacher sells a 1.5A fully automatic onboard battery charger/maintainer for around \$25. It's biggest problem is that it stops charging at 14v, and starts charging when it falls 1v below that.

That would be OK in the north, when the battery internal temps might be from 0°C to 10°C, but will overcharge batteries when they're warmer.

The charger Brian's looking at has temperature compensation available as an option; very necessary in my opinion.

7. studiot AAC Fanatic!

Nov 9, 2007
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Thanks for the extra information.
I don't have much experience of accumulator cells under extreme conditions.