# Charging a lead acid battery with a lower bulk voltage

Will a lead-acid battery completely charge when using a bulk voltage below the one indicated on the specs sheets for it, even if it takes longer?

In my case the battery is in a place with temperature fluctuations, and would be safer to have a lower charging voltage when is warm.

• You want to lower the generated heat by reducing charge current? Sep 25, 2018 at 9:42
• No, for what I read, the charging voltages should be lowered when temperature rises above 25ºC. I only want to know if I set a lower voltage the batteries would attain a full charge (even in lower temperatures). Sep 25, 2018 at 13:05

Nearly all cars have a Lead-Acid battery. In a car huge temperature fluctuations occur (engine on / off, car is outdoors). Lead-Acid batteries are quite robust and can handle some overcharging. As far as I know, in a car the charging voltage isn't changed with temperature and still most car batteries last for years.

As far as I know, what is advisable for an increased battery life is to not charge above 13.6 to 13.8 V (see battery's datasheet). If you then also limit the charging current (a series resistor will do that) as the battery voltage increases the current decreases as well and no damage should occur.

Instead of changing the voltage (which would be a challenge to get right as the battery voltage is quite unpredictable) it is better to limit the current to for example C/10 (where C is the battery's capacity in Ahr, so for a 1 Ahr battery C/10 = 0.1 A) then even when the battery voltage is low due to temperature, the charging current is limited and causing no damage.

Charging a battery isn't just about the voltage alone. There's 3 stages of charging:

1. constant current
2. topping charge
3. floating voltage

Quoted from Battery university

The constant-current charge applies the bulk of the charge and takes up roughly half of the required charge time;

the topping charge continues at a lower charge current and provides saturation,

and the float charge compensates for the loss caused by self-discharge.

The temperature will have a drastic rise in stage 1 due to the bulk charging, which is mainly due to the high charging current. This is if you're using a smart battery charger and not just pumping DC to the battery from a DC source. Some sophisticated battery charger do take temperature measurement of the battery and will pause the charging to prevent the battery from catching fire.

You mentioned that your battery is warm when charging but that's a normal phenomenon when you're charging a battery. From this link you can see that it's ok for the battery to rise up to 50°C. If your battery isn't a sealed type then you need to open up the valves for the generated gas to escape, and you should charge the battery in open-space with air ventilation.