In this article we will discuss the La Marche method for sizing a battery charger. There are a few pieces of information that must be gathered before the charger size can be calculated. The required information is as follows:

– What is the maximum load in amps?

– What is the battery backup time needed?

– What is the required recharge time?

– What is the battery type (Lead acid or Ni-Cad) and the number cells?

Once we have the required information we apply the following Battery Charger sizing formula:

A= (EF x AH/ H) + L

A = DC output rating of charger

EF = Efficiency Factor - used to return 100% of amp hours removed from a discharged battery. Recharging a battery is not a 100% efficient process. So it is necessary to put in more energy than we took out of the battery. This is the purpose of the efficiency factor in the charger sizing formula. La Marche uses the following efficiency factors:

1.15 - for lead acid batteries (Flooded and VRLA)

1.40 - for NiCd batteries

H = Recharge Time – Expressed in hours, this is the desired time to recharge and bring the battery back to full charge capacity and it varies by application. Typical values are 8, 12 or 24 hours. The battery manufacturer should be consulted recharge time faster than 8 hours or longer than 24 hours is required.

AH = Amp Hours Removed, calculated number of ampere hours removed from battery. Many people use the battery amp hour rating of the battery to size the charger. While you could use the actual battery amp hour size to calculate the charger, the proper method is to use the actual amp hours removed from the battery. You can either estimate the amp hours removed or it is calculated automatically when you use one of the many battery sizing programs provided by the various battery manufacturers for their batteries. For continuous loads, a quick way of calculating the amp hours removed is by using the following formula:

AH = L x BT

L = Continuous load - Many times the charger has to carry a continuous load while recharging the battery. In these situations this load is added to required amps to recharge the battery. Loads that are short in duration or transient are handled by the battery and not the charger.

BT = Backup Time - Expressed in hours, this is the time required for the batteries to support the load. 

Charger sizing example:

We have a customer requiring a battery charger. He tells us that he has a piece of equipment he needs backed up with a battery and charger. The load is 25 Amps @ 24VDC and he needs it backed up for 5 hours. He also wants the system recharged in 8 hours. He plans to use a 12 cell lead acid battery.

First we need to figure out the estimated Amp Hours removed using the following formula:

AH = L x BT = 25 x 5 = 125AH

L = 25 Amps

BT = 5 Hours (Back up time)

Now that we know the estimated Amp Hours removed we can continue to calculate the charger size using the following formula.

A = (EF x AH/ H) + L = (1.15 x 125/8) +25 = 42.97

A = DC output rating of charger

EF = 1.15 for lead acid battery

AH = 125 amp hours removed

H = 8 hour recharge time

L = 75 amp Continuous load on system while recharging the battery

Round up to the next common size use a 50 Amp, 24VDC battery charger.

If you have any requirements or any kind of query regarding the battery charger solutions, feel free to communicate with our dedicated team at any time at marketing@everexceed.com.