MAILBOX: Battery-Charging Idea

Some of (now retired) Ralph's products.

Now that I’m retired from Bogart Engineering, I’m writing this to get reader reaction to a method I’ve thought about for awhile and have started to implement. I’m also writing because I want to put this idea into legal “public domain” so it can be used by anyone, without patent obstacles. After 25 years of living with an off-grid PV system, I’ve decided that the better way to charge batteries is by using a two-day charge cycle instead of just one day.

It’s often thought that to charge a battery in one day, all you need is plenty of PV capacity—modules and sunshine. Of course, the chemical process of converting electrical energy to chemical energy requires enough amp-hours (Ah). But often people don’t realize it’s important to have enough solar time to charge them well. Lead-acid batteries—especially at the end of charge—absorb energy more slowly. One day just isn’t enough time to completely charge them to preserve capacity. That’s a fundamental limitation of the battery chemistry—similar to the logic that you can’t bake a cake in half the time just by increasing the oven’s temperature.

My idea is to use a two-day charging cycle: For example, instead of a 600 Ah capacity storage, divide the system into two 300 Ah battery sets. Discharge only one set the first day. The next day, switch to the other set, but continue charging the first set at a low finish-charge level. At the end of the day, switch to the well-charged set. This makes a two-day charging cycle using both sets.

It’s generally regarded as good practice to discharge an off-grid system only to 80% SOC (state of charge). I’m now advising to discharge the “half-sized” system to 60% SOC—beyond what’s normally recommended. How can discharging twice as much possibly be beneficial?

For the battery’s health, leaving the smallest amount of uncharged lead sulfate per day as possible is good practice. If, in one day’s charging, 5% uncharged lead sulfate is left, and, after charging for an additional day (without discharging), only 5% of 5% (or 0.25%), which is 0.125% per day, is left, that is a 20-fold reduction in unreacted residual per day. The result is that equalization cycles may not be needed as frequently.

With a typical one-day charging cycle, the productive afternoon sun can’t be used efficiently because finish charging of the battery in the afternoon must be done at low amperage. With a two-day cycle, this lost solar energy can be accepted by the batteries. I estimate that this method could use 30% more performance from solar-electric arrays. Stay tuned for an update after I gather some data from my test system.

Ralph Hiesey • retired founder of Bogart Engineering

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