# Variable Current Lead Acid Battery Charging

I'm designing a solar controller in order to charge SLA batteries.

I did some research on dedicated ICs that act as controllers, and found that most implement some sort of MPPT, in order to extract the highest amount of power.

Since the power that the PV panels give can vary in time, I suppose that the charge current isn't a constant but it depends on the conditions. How does this non uniformity of current impact on the battery life, compared to an ideal CC-CV charge curve?

Would it be better to disable the charge if the panels can't provide the power required to satisfy the set current (like an on-off control scheme)?

If anyone has any extra information, I'd appreciate it. Thanks!

• The CC charge is just a safe maximum. You can safely charge at any smaller current, it will just take proportionally longer. If anything, the batteries should last longer with a slightly slower charge. The CV is a fixed limit, and this is where you can make a difference to lifetime - floating fairly low, occasionally charging it high, quick bursts of discharge, etc. Commented Apr 12, 2015 at 5:28

The short answer is - NO! No need to waste power from the panels.

You are correct, the lead acid batteries are best charged on a CC-CV methods. However, constant current doesn't mean that the battery has to be fed an exact pre-determined current or the battery suffers. It would be better interpreted as a maximum current that the battery can be charged at.

In an ideal world, where lead acid batteries worked better, we would just apply the voltage at the level of the constant voltage and let the current flow in until it saturated. Unfortunately, if we did that with lead acid batteries currently being produced, the current flow at lower charge levels would be so high that they would fry the battery. As a solution, the charge starts at a constant current level that is lower until the battery starts to fill up to the point where you can apply the constant voltage voltage and the current at that point won't be too high because the battery is already fairly full.

So if that made any sense, you should be fine to charge the lead acid battery at a lower and uneven current in the constant current stage of charging as long as the current doesn't exceed what the manufacturer recommends for the battery that you are using.

Specifically to answer your question, charging at lower current doesn't hurt the battery (unless the current is so low that the battery never charges because self discharge is greater than the charge current!).

Would it be better to disable the charge if the panels can't provide the power required to satisfy the set current (like an on-off control scheme)?

I agree with Filek’s short answer: Do NOT DISABLE the charging.

Battery chargers use the Constant Current limit (CC) during the Bulk charging phase, but that is considered an Upper limit, associated to the battery size and battery capacity to receive current and convert into electrochemical charge.

A classic 3-stage battery charging system is shown as here (adapted from here):

However the power from a Photovoltaic “PV” panel is not constant, and varies with the amount of instantaneous solar power available.
Researching commercial Solar Charger controllers, the user manual of this one from Schneider Electric, brings useful information, per following pictures - about MPPT tracking:

And about the 3-stage charging method being used as a reference, but the blue boxed text shows the actual charging parameters depend of Solar power availability:

I also found a remarkable source of information about MPPT and CC-CV algorithms and methods for a smart asynchronous buck-converter and MPPT controller, entirely made by TechBuilder and shared on a Instructables page. The downloadable PDF version has 156 pages and there is also a video showing the design & construction of the MPPT Charger, that I recommend to be seen before the reading.
Inside the TechBuilder’s project, he mentioned he developed the variable charging algorithm using his modified version of “Coulomb counting” method.

I also found two articles discussing and reviewing different techniques of MPPT and monitoring the State-of-Charge “SoC” of batteries. The links are here and here. It gave me a better understanding, not commercially biased, of existing SoC and MPPT techniques.