A question on Lead Acid Batteries

When bringing up batteries to full charge, overcharging is standard practice. While on constant float, it does not get that overcharging which helps get rid of sulfation, and increases storage.

greg

I have a question regarding lead acid batteries, specifically regarding the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the batteries at a float voltage as opposed to letting them self discharge for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a readiness state, which is why I suggest only letting them self discharge for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages of the two methods. Does anyone have an insight or experience that may help me decide which method to use?

My understanding is that for battery health, either float or isolation with periodic recharge are both approximately equally acceptable.

In either case, you will need to initially fully charge the battery, so the equipment for that will be equivalent.

After that, float is MUCH easier to do automatically ...

to float, you hook up to a constant voltage source and you're done.

with periodic recharge, you have to have some kind of timer or voltage sensor, a way to connect to the battery,. a way to tell when the battery is charged and disconnect again.

If you have parasitic loads connected to the battery, it doesn't affect the float scheme at all (assuming the float source can handle the extra current) The periodic recharge scheme would perhaps need the float interval length adjusted.

With both flooded lead acid and Gel or AGM lead acid batteries there is a slight advantage with constant float charge as it prevents the build up of sulphation. Each time the battery is discharged, no matter how little, and no matter whether it's by way of a load or self discharge, some of the sulphate remains as sulphate even after a full recharge. This is just a part of the natural battery ageing process.

Maintaning a constant float charge reduces the extent of this effect dramtically over the extent when the battery is allowed to stand then recharged.

With wet cell (ie flooded) lead acid batteries there is a slight problem in that during float, the electrolyte stratifies and the first discharge cycle after an extended float does not produce the full battery capacity. This effect doesn't happen with periodic recharge following a few days with no charge. Well it does happen during the standing period, but the following full charge solves it.

For this reason many advanced chargers run float with a periodic full charge at some interval between about 48 hours and 28 days. Different manufacturers have different ideas what works best.

As regards battery life, the difference between the two methods is actually quite minimal. But the effect of the reduced first discharge following extended float may sometimes cause problems.

With AGM and Gel cells there is no doubt that maintaining a constant float is the better option.

We use the method of float charging with a constant regulated 27.6V (2 series 12V gel type) We have encountered no problems, some battery manufacurers recommend a salightlu lower voltage for float charge but as I said we've had no problems with approx 2000 units out in constant use. We do recommend replacing the batteries sooner than battery manufacturers suggest, but the application is a fire safety system and needs to be reliable.

First off, I would like to thank everyone for taking the time to reply to my inquiry. The responses have given my some ideas to consider on both sides of the fence.

This application is also part of a fire safety system and reliability is my foremost concern, which is why I can make arguments for both ways. One of my biggest areas of concern is float voltage versus the temperature, which can vary wildly with different installations.

As it stands, I am thinking about defaulting to a float condition, but leaving the option for a periodic top off charge instead as a programmable / selectable feature.

We do the same sort of thing. A current limited bulk charge with a float at the end and you can leave it floating if/all you want. Lots of units in the field, no problems. I went with the classic 13.7V for the float, but with resistor tolerances it can go +/- .1V anyway.

Jim

It wouldn't do any harm to monitor the batteries' temperature, albeit it would add complexity to your circuit. But a battery with a float voltage that's adjusted for temperature is a very happy battery. :-)

Good Luck! Rich

The TI chip I used several years ago, took all that into consideration. It was a Unitrode chip and just need a pass transistor. greg

The TI chip I used several years ago, took all that into consideration. It was a Unitrode chip and just need a pass transistor. greg

The TI's I've seen need a bit more than that. Resistors to set the bulk charge rate, peak voltage, float voltage, and so on. The UC3906 is a prime example. Nice IC's use them in our upper end stuff, but they do need a bit more than a pass transistor.

Jim

discharge

Indeed they do, but IMOE they are one of the best SLA/gel charge controller chips around. I highly recommend the O/P read the TI/Unitrode appnote U-104 for them, to be found at: focus.ti.com/docs/prod/folders/print/uc3906.html