Battery Charging Voltages and Temperature

electricity from the sun by John Drake II | home

A lot of folks believe that very high battery charging voltages allow you to get the most out of

your batteries.

Some battery manufacturers recommend very high charging voltages to "top off" their batteries.

Before we get into my biased opinions let me tell you a story.

Many years ago Light It Technologies (Montana Light) came out with some high output led lights

with an Edison base (standard screw in base).

These were offered in either 12 or 24 volt models.

The 12 volt jobs worked like a champ because most 12 volt photovolatic charge controllers

could not be set at a high charging voltage.

The 24 volt models started coming back for warranty repair.

After asking a lot of questions, and seeing what components failed, they came up with

some surprising revelations.

Many customers had set their charging limits well over 30 volts to "get the most" out of

their batteries and since the led light power draw was so small, they left them on all of

the time.

Well, some were being damaged by "normal" charging and all were being destroyed

when the batteries were having an equalization charge.

LIT repaired the lights and returned them with a warning to shut them off during the day.

Well, people did not heed their warning and were smoking the lights and returning them

for repair.

LIT dropped the lights in 24 volt and refused to make them because of the losses

they were taking.

You want to consider that when you are charging your batteries at a high voltage,

what is it doing to the devices you have connected to your batteries.

Also, your batteries generate heat when being charged.

High battery temperatures can shorten battery life.

Let's not forget that high charging voltages will cause the batteries to lose

more water - something to consider.

In an alternative energy setup batteries can usually be placed with a good air gap

between each battery.

I always recommend at least one inch between each battery to allow for cooling.

In an rv or boat, this can sometimes pose a problem due to limited room in the

battery area.

If you don't have plenty of room to allow the batteries to dissipate heat, you

might give some serious thought as to what you want the

charging limits to be.

Always wear eye protection when working

near batteries.

Even a small battery stores a great deal of

energy which can be released during a

short circuit.

The electrolyte contained in batteries is

corrosive.

The following information is courtesy of Ralph Heisy of

Bogart Engineering.

Battery voltage will vary for the same state-of-charge depending on whether

the battery is being charged or discharged and what the current flow is in

relation to the size of the battery.

The chart below will give you an idea of state-of-charge for various battery

conditions in flooded cell lead-acid batteries.

Voltage varies with temperature.

While charging, a lower temperature will increase battery voltage.

Full charge voltage on a 12 volt battery is 0.9 volts higher at 32 deg. F.

While discharging, a higher temperature will increase battery voltage.

There is little temperature effect while a battery is standing.

The following information is courtesy of

AEE Solar.

A hydrometer is very accurate at measuring battery state-of-charge if you measure

the electrolyte near the plates after it has been at rest.

Unfortunately, you can only measure the electrolyte at the top of the battery.

When a battery is being charged or dis-charged a chemical reaction

takes place at the border between the lead plates and the electrolyte.

During charging the electrolyte changes from water to sulphuric acid.

The acid becomes stronger and the specific gravity rises as the battery charges.

Near the end of the charging cycle gas bubbles rising through the acid stirs the

fluid to mix it.

It takes several hours for the electrolyte to mix so that an accurate reading can

be taken at the top of the battery.

Always try to take readings after a period of no charge or no discharge.