# Charging a 24Volt dc lead acid battery

Discussion in 'The Projects Forum' started by Landon, Jan 13, 2009.

1. ### Landon Thread Starter New Member

Oct 3, 2007
9
0
Hi guys, I'm relatively new here and i require some guidances from you all.
I would like to find out the 'rules' regarding charging a dc battery eg charging a 24 volt lead acid battery rated 100 ampere hour. I understand that the input to charge a 24 volt battery should be higher but by how many percent? Need your enlightenment.

2. ### Bernard Expert

Aug 7, 2008
5,225
600
There is lots of information on 12V batteries,just double figures , if 2 12V batteries charge singley or in parallel.My motor home uses 2 12V in parallel even if it isnt the best . Some figures: Assuming flooded cells, charging V 28.4-29.2 ; float V 26.4; charge @ 20% AH ie. 20A for 100 AH.max. Fully charged,resting, 25.2-25.4,spec. gr. 1.265; 24.8V @ 1.225 =75%; 24.4V @ 1.155 = 50%; 23.4 @ 1.120 = discharged. To equalise ,2hr@31V. Parts from " Trojan rider".

3. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
TM 9-6140-200-14, a battery maintenance manual created by the Army, recommends a maximum charge rate of 5A for high capacity 12v batteries, such as the 6TMF 12v 120AH battery.

Charging the battery at a higher rate will cause the internal temperature to rise, which decreases the lifespan of the battery. Batteries last much longer at 60°F than they do at 100°F. It takes them a considerable amount of time to cool down once they are heated up.

If a 12v battery discharges to 12.5v @ 25°C (77°F), plate sulphation will begin. This is the #1 cause of early battery failure. For you, this is 25v @ 77°F, but it needs to be adjusted according to the battery internal temperature. You can check the internal temperature by measuring the positive terminal's temperature.

There are three basic phases to charging a lead-acid battery:
1) Bulk charge - the battery is charged at a constant current until it's 80%-90% charged.
2) Absorption charge - the battery is charged at a constant voltage (roughly 2.33v per cell, or in your case about 28V) until it is nearly 100% charged.
3) Float charge - the battery is charged at 2.26v per cell at 77°F (in your case, 27.12v) indefinitely.

For maximum battery life, the voltages need to be adjusted according to the internal temperature of the battery.
It is a negative temperature coefficient; 0.003v per cell per °C below 25°C, -0.003v per cell per °C above 25°C. Since your battery has 12 cells, that's 0.036v per °C.

4. ### Søren Senior Member

Sep 2, 2006
472
30
Hi,

An important point...
Is it a motor start battery or a deep cycle battery?

5. ### jj_alukkas Distinguished Member

Jan 8, 2009
753
8
How can we measure that?? Direct temperature sensing??

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Yes. An LM34 or LM35 attached to the terminal makes for a simple analog solution.