It is not all that hard to calculate what the boost/absorb time should be, and the equation actually makes sense when looked at a little closer. As it turns out, absorb time should generally be quite a bit longer than most people would think. First, here is the equation for absorb time:
Absorb-time (hours) = 0.42 x Battery-Amp-Hours (Ah) / Charge-Current (Amp)
Let's do an example that shows how it works. Say we have a smallish system, consisting of four solar panels of 300 Watt each, a 24 Volt battery bank made out of four Surrette S-550's in series (each 6V @ 428Ah - that is the 20-hour rating), and an EP-Solar/EP-Ever 40A MPPT charge controller.
So what is the charge current going to be?
Current (Amp) = Power (Watt) / Voltage (Volt)
Realistically, a 24V battery bank charges at around 28 Volt (or even higher), and we have 1,200 Watt of solar panels. So this works out to:
Current = 1200 / 28 = (nearly) 43 Amp
Of course, the charge controller is limited to 40A and will clip at that value. Now, in reality panels rarely produce rated output, and the sun rarely shines full-tilt, so for battery absorb purposes I use 85% as a more realistically available current:
Current = 0.85 x 43 = 36.5 Amp
We have all the variables for calculating the absorb time now, so let's give it a whirl:
Absorb-time = 0.42 x 428 / 36.5 = 4.9 hours (!)
That is 4 hours and 55 minutes, yeah, it really takes THAT long! The EP-Solar charge controllers won't even go for that long, their maximum boost time setting is 3 hours.
So where did this equation come from you ask?
When lead-acid batteries are being charged with the usual boost-absorb-float cycles, the first phase, boost, takes them up to about 80% - 85% State-Of-Charge (SOC). During boost charging the controller puts as much current into the batteries as it can, so the battery Voltage slowly rises, until it reaches the absorb/boost Voltage setting and the boost-stage is over.
At that point we still have 15% - 20% to go before the battery is actually "full". Or in Amp-hours from our example, we still need to put back 65Ah - 86Ah.
From our example we had a realistic current at this point of about 36.5 Amp. If we could put back 86Ah efficiently that would take just 86 / 36.5 = 2.3 hours. But life with lead-acid batteries is just not that good! Putting those back is not going to be efficient, at that high a Voltage we are spending an awful lot of energy electrolysing water (hence the vigorous bubbling of those batteries!). We will have to try harder! Or, as the case may be here, longer.
At the same time, during absorb charging, the charge controller keeps the battery Voltage constant and tapers down the current as needed, so that Voltage stays the same. In other words, we don't even have 36.5 Amp. It's getting less and less as the absorb phase goes on!
If the current tapered linearly (and it does somewhat but not quite do that), it would take twice as long to put back those missing Amp-hours. The factor in the equation would then be somewhere around 0.3 to 0.4. Trial & error (my favourite scientific method!) has found that 0.42 is a reasonable number to get the job done.
I realize most people's eyes glaze over when they see an equation, but let's take a closer look at that absorb time one:
If you divide the Amp-hour capacity of the battery bank by the current going in (in Amp) you get the time it would take to re-charge the batteries from empty to full, if the process was 100% efficient. So, 428 Ah and 36.5 Amp makes nearly 12 hours. We only need to put back 15% - 20% during absorb, so that makes 1.8 - 2.4 hours. But that charge current is tapering off, making it so we need twice as long (exactly twice as long if the current tapered as a straight line to zero and charge efficiency is 100%), so that time becomes 3.6 - 4.8 hours. Inefficiencies in the charge process make it so we're at the high-end of that time, and then some, 4.9 hours of absorb time. That is where the 0.42 factor came from.
Clearly, the bigger your battery bank, the longer it will take to recharge. That is why "Battery-Amp-Hours" in the equation is something we multiply with. Twice the battery bank size, twice as long an absorb time. Makes sense, no?
Similarly, the more current we can pump into that battery bank, the shorter it will take to recharge those batteries. That is why the "Charge-Current" in the equation is below the division line. Twice the current, half the time it takes to absorb. Sounds reasonable?
Hopefully the equation now makes more sense, and you can use it to calculate the absorb time for your battery bank!
Now, is it really all that important to exactly stick to this absorb time number? No, it is not. Reality with batteries is a little more complex and the equation is just a simplification to get us a number that works well enough. Absorb time is also determined by the charge Voltage, or more specifically the boost/absorb Voltage. A higher absorb Voltage "works harder" and puts more energy back, possibly allowing for a shorter absorb time. It is also very much a factor of the state of the plates of the battery, how 'sulphated' they are. In some ways the equation really is about worst-case what it takes to reliably get those missing Ah's back and clean sulphate off the plates (to a point, there comes a time when sulphate crystals grow to become nearly irreversible!). As with all in life it is a trade-off, but one that works for most to keep their batteries happy.
I know that most people don't use anywhere near the absorb time this equation would suggest, and their batteries are fine! Yup! That is entirely possible. Especially if batteries get cycled relatively shallowly (and anything less than 50% SOC is 'shallow'), shorter absorb times may work just fine. So don't obsess about this, do what you can. If your charge controller allows for the full absorb duration, set it (so it uses it when the sun is there to make that happen). If not, just set it as large as you can. Your batteries will thank you for it!
-RoB-
