How to Charge Your EV’s AGM SLA Batteries


This is a work in progress- I am working on planning how the battery regulators for the car are going to work. One key part of that process is deciding on the actual optimal charging procedure for the Optima-type AGM battery.

One often-seen charging procedure is the “three-stage charging” process. It consists of three distinct phases:

  1. Bulk, constant current. The charger puts out its maximum possible current until the battery voltage reaches a certain setpoint (usually 14-15v for 12v nom. batteries.
  2. Absorption, constant voltage. The charger then holds the battery at that setpoint as the current declines, either for a certain amount of time or or until the current drops below a certain setpoint.
  3. Float, constant voltage/current. The battery voltage is kept at a lower voltage (usually around 13.8v) where there is little change in the current.

This process is recommended for its speed and efficiency. It would be reasonably possible to implement with the battery regulators.

The Optima documents specify a different regimen, however, for cyclic applications (applications like in the car where the batteries are cycled significantly, often- as opposed to starting or UPS use)

Cyclic Applications:

  • 14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below 1 amp, finish with 2 amp constant current for 1 hour.

This is a bit different, and a bit strange seeming. I found the Exide Orbital tech sheet a little more descriptive and useful to clarify and reinforce this different idea:

2. IUI or Constant Voltage/Constant Current/Constant Voltage Recharge: Appropriate for more continuous
cyclic discharge/recharge applications
o Step 1: Maximum current limit of 20% of nominal capacity (approx: 15 amps) and voltage limit of 14.4 volts (A)
o Step 2: Hold at 14.4 volts until recharge current reaches 2% of nominal capacity (1.5 amps for most Orbital sizes)
o Step 3: Hold current at 2% of nominal capacity (1.5 amps) for 2 hours
o Discontinue charge if battery reaches 50oC (122oF)

Taken from Exide Orbital Technical Information PDF

While the numbers Exide chose for the Orbital charging recommendations are a bit different, the concept is very much the same. It’s definitely useful to see this kind of consistancy.

So, how will this work?

The individual regulators can be aware of the voltage of the battery they’re regulating and the current they’re sinking in the shunt by way of ohm’s law and the duty cycle the shunt is running at. The regulators must be able to be aware of the current the charger is putting out for a constant current mode. Temperature compensation capability would be desirable. The master charger regulator must be aware of the duty cycle of the regulators and the voltage they’re sensing to decide how to scale back the charge current. Alternately the regulators could ask the master to reduce current, although the latter setup could be less versatile.

I think that accounts for all the provisions laid out by the Optima charging specs, and this hardware setup. I’m sure I will continue to think about it and how it’s going to work though– of course!

Posted on May 22, 2009 at 2:14 am by Henry · Permalink
In: Battery Regulator, EV Miata

Leave a Reply