Choosing the correct size Battery Charger

If you want to live off-grid, you need to get one thing right first: your battery charger. So, how do you know which size charger you need?

Work out your charger size based on the following factors:

1. Battery Type

For lead acid deep cycle batteries (AGM or Gel), you want a charger with an Amp output of 20% of the total battery bank. So, for a 100Ah battery, you would buy a 20Amp charger.

Different battery types have different requirements for charging. That’s because the battery chemistry reacts differently when being charged.

For Lithium and Lead Crystal batteries, the charger to battery ratio is based on 30% for maximum performance and lifespan. That means a 100Ah requires a 30Amp charger.

Off-grid Collective Solar Battery

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2. Charge Profile

This includes the Maximum Charge Voltage and Maximum Charge Current.

The Maximum Charge Voltage is the highest acceptable charge voltage the battery can take to charge.

This Maximum Charge Current is highest charge current that the battery can receive when charging.

You must charge the battery within the specifications of the maximum charge voltage and charge current. If not, you can severely reduce the battery’s health and life.

A 200Ah Lithium battery will have a maximum charge voltage of 15.5V and a maximum charge current of 150A. The charge profile must suit these specifications.

3. Multiple Charging Sources

If you have the ability to run multiple charging sources at that same time (like in a motorhome or boat), these should not exceed:

  • 35% of the battery capacity combined for Lead Acid/AGM
  • 30% for Lead Crystal
  • 60% for Lithium

The good news is a lot of chargers have the ability to dial down the current output of the charger. This allows for installing a bigger charger at the start, which is ideal if you think you may increase your battery capacity or number of battery banks at a later date. No need to buy another charger down the road and waste money.

Another thing to note, is that when powering your battery charger from a generator you need to ensure that the battery chargers power draw doesn’t overload the capacity of the generator.

4. System Voltage

Your charger output needs to be suitable for your system voltage.

If you have a 12V system, choose a 12V battery charger.

For a 24V system, you need a 24V battery charger.

Different Types of Battery Chargers:

AC to DC chargers are designed to charge your battery from a generator or mains power. So, rather than using your generator’s 12V supply, use a regulated charger and plug it into the AC source on the generator as it provides faster charging and wastes less fuel. That’s a big benefit if you’re in a remote spot, hours from the nearest fuel station.
A DC to DC battery charger is like a smart charger for your 12V system. The best thing about a DC to DC charger is that it will boost charge into your battery while you’re driving. It also acts as an isolator, so your starter battery doesn’t go flat when you stop for a while.

Victron Easy Solar Battery Charger

Battery Management Systems

Another option is a battery management system. This provides a complete system for charging auxiliary batteries, so you can be confident they’re always fully charged.