constant volts per hour charge controller

[joestue]

Looking at accepted literature for lead acid battery terminal voltage vs state of charge, i believe that a simple charge algorithm can be developed simply by modulating the current into the battery to maintain .25 volts per hour increase. such a method will provide the highest currents between 40% and 80% SOC, tapering off significantly as the battery approaches end of charge. further logic would of course be required to terminate the charge.

I'm fairly certain I can build the circuitry with just a few opamps and some low leakage capacitors, and make it bug free such that it is fool proof.

at the moment i'm proposing a boost converter that would require the brains to operate without which the voltage will be too low to charge the battery.. much safer than an unregulated transformer with a failed buck converter boiling batteries away.
but the system would work with buck converters as well.

any thoughts?

[boB]

Interesting...  Why did you pick 0.25 V/Hour ?

Is that a 12V battery ?  24V ?  48V battery ?

That would be 1.0 volt per 4 hours.  If it is a 48V battery it will never get charged in
one day and you will need an absorb cycle for 2 to 4 hours once in a while as well.

What happens when you have say, clouds that keep you from increasing by 0.25 V for say, 2 hours
and you only have 2 hours of sun left for the day capable of raising the voltage by that amount and
you need your batteries charged today ?  Sometimes it is hard to predict the weather.

I would limit the charge amount more from battery temperature rise, maybe.  Charge by an amount
of current that keeps the battery temperature from going over a certain temp limit as well as limiting
the current to somewhere around C/5 or whatever the recommended maximum charge rate is.

boB

[OperaHouse]

I have felt cosmic angst a I have developed software for my charge controller.  While wanting to have more sophistication, experience suggests that most systems never get out of bulk charge.  Just don't think this is an issue unless your system is way oversized.

[boB]

I have felt cosmic angst a I have developed software for my charge controller.  While wanting to have more sophistication, experience suggests that most systems never get out of bulk charge.  Just don't think this is an issue unless your system is way oversized.

Yep, this is often the case.

The batteries must go into Absorb once in a while otherwise they will sulfate and die an early death.

boB

[joestue]

I was thinking half a volt per hour would be better but that would result in very fast charge rates.
That would be for a 12v battery. 24, 36 and 48 would need to be scaled up 2,3,4 fold.

And of course this concept of limiting the charging rate must be factored into whatever power sources are available.
for solar and wind it would make no sense.

I am more interested in the safe and reliable charging of a random lead acid battery of unknown capacity and unknown state of charge, primarily from the grid.

[dnix71]

There used to be a nicad charger (30+ years ago) that worked sort of like that. It simply applied 1/2 volt above what the pack had until the pack voltage stopped rising. That prevented damage to the weakest cell. It also meant that if there was a marginal cell, the whole pack would be undercharged.

1/2 volt is not enough to ionize water, so unless the pack overheated from current alone there was no water lose through hydrolysis.

[Flux]

For mains charging this may work. You have realised that it can't work with any intermittent form of charging.

Ideal charging from a mains supply is reasonably easy, it is far more difficult to keep track of things once you get out of bulk charge with opportunist charging.

I think the equations for the lead acid battery can be computed with reasonable accuracy( the smart gauge seems to work) but your simple approach may be good enough as long as you don't interrupt the charge cycle or even worse introduce any discharge.

Flux