# 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:

- 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.
- 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.
- 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?

- At first, all the regulators will not shunt, voltage will be under 14.7 volts at all batteries and current at all batteries will be the same, up to 20 amps (charger limit)
- As some batteries start to approach 14.7v the regulators will start regulating, up to their maximum shunt current, at which point the master regulator will start scaling back the charger current. Batteries that have not reached 14.7v yet will continue to charge at the lesser amperage. Charger current will continue to decrease to avoid pushing the strongest batteries over 14.7v.
- The charger output will go as low as 3.5 amps, since the regulators sink about 2.5 amps at wide open, that will allow the strongest batteries to reach 1 amp at 14.7v while the weaker ones continue to charge at up to 3.5 amps
- Once the strongest batteries have reached 1 amp at 14.7v the regulators can go into “extended absorbtion” phase for those batteries, with the charger still at 3.5 amp output can hold those batteries at 2 amp constant current for an hour, while the weaker batteries continue to do whatever they need to do.
- Once the strongest batteries have been charging at 2 amps for an hour, the charger will reduce to 2.5 amps to allow those batteries to stop charging entirely or 2 amps if all the other batteries have reached “extended absorption” phase. The regulators on those batteries that have completed an hour will shunt all the charge current.
- A low current float phase could be implemented after all batteries have completed an hour at 2 amps.

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!