An interesting article on charge efficiency

[booster]

Most battery meters and some chargers use charge efficiency to control and display things that we can use for keeping track of our batteries. Without factoring in charge efficiency, the units would have lots of trouble being even close on recharge cycles for SOC. It appears to be the Peukert factor of the recharge side.

As I have been messing around with our new (and old) stuff, it has gotten very obvious that how I understand charge efficiency isn't really how it works, unless I am missing something by a long shot (always possible for me).

All the meters and our Magnum charger reset the SOC display to 100% on the charge side once they meet certain parameters like time at voltage, return amps, total amps recovered, etc, regardless of what the unit has calculated the SOC should be during recharge, which is calculated by the charge efficiency. Basically, they do that reset because the calculated state of charge % could be way, way off. If you don't get full to the parameters to do a reset it will get further off each cycle.

I couldn't get consistency on the Magnum, which recalculates the charge efficiency on each cycle reset. Each time you recharge it is putting a different charge efficiency factor on the amps back calculation that is based on previous (how many?) recharges. Sounds like a good way to do it over time, except that after a bunch of reading I found out the charge efficiency changes huge amounts based on the depth of discharge, so you are trying to hit a moving target, if you don't have consistent discharge amounts.

Here the best article I have found so far on the variations.

http://www.otherpower.com/images/sci...Efficiency.pdf

It basically says that at low SOC you might have 90%+ charge efficiency and at high state of charge it might be only 20% efficiency.

So if we ran a bunch of 10% down cycles our charge efficiency would go to the very low % automatically. If we then did a deep discharge, where the overall efficiency would be in the 80% range, or SOC meter reading would be hugely off until it reset.

The Trimetric does not recalculate charge efficiency, it is settable and is set at 92% default. The reset requires it to get to a settable return amp reading, so the resets will be very accurate for the next discharge cycle. The recharge cycles can be way off, though, depending on the depth of discharge, I think, because of the charge efficiency variations.

Magnum has their preferred method of transitioning from absorption to float based off the SOC calculation on recharge. They suggested I use it because it does go into bulk at every plug in, which I want. It looks at all the same stuff as the Trimetric, it appears, but the critical one to getting full batteries (return amps) is not settable and is fixed at 2%. This is way too high for the Lifelines that want .5% or even our Trojan 6 volts that are just under 1%. Their second requirement for transition is that the SOC gets to 100%, but this number can be completely off, I would think, based on the change in charge efficiency from the depth of discharge. I have tried several test cycles of using the SOC transition, and they have been off by as much as 40AH of returned charge (obtained by doing a return amp charge right after the test run).

Unless I am missing something, I don't think there is any way that the SOC transition could be very accurate on the Magnum, unless you happened to need exactly 2% return amps, or always had the same discharge depth. If the 2% were settable, it would work extremely well, I think.

I also read that charging rate affects the charge efficiency, so that those of use with 3 hugely different charge rates (shore, engine, solar) would have even more things mucking up the SOC reading on the meters and chargers.

It looks like we will be using the return amp transition to float setting on the Magnum, even though we will have to force it to bulk each time we plug in if the voltage is up. The test runs I have made using the ending amps have been amazingly accurate and repeatable.

To those that have Trimetric meters, I would recommend taking a look to make sure the return amps are set right, so you get accurate SOC resets each time you get full. After you get the return amps right, mess with the charge efficiency to get it to match your use patterns as well as it can, so it reads closer on recharge cycles. If you are just pushing the manual reset of the SOC each time you think you are full (easy to think you are full when you are not because most chargers stop early), there is a very good chance you are way off on your actually SOC vs the meter reading.

[gerrym51]

Booster- i have been toying with idea of adding another tppl agm battery to my zion. do you think it would have to be same size.

if all batteries are 12 volt is the fact they are not the same size when charging an issue?

just wondering

[booster]

We have mismatched batteries in our current system, and it is a bit of a PITA to deal with. They charge and discharge at different rates so it is very difficult to keep everything in balance and get full charges on them.

Everything I have read, and experienced, would indicate the batteries should be identical, right down to the age (some would even say out of the same production run).

So you would want to get the same manufacture, same identical part number, for sure. You would still have the mismatched ages, however, which would compromise a bit. You may also want to see my other thread on cable length matching, as that can also make a difference.

[booster]

I found this chart of state of charge vs charging efficiency in a spec sheet for smaller AGM batteries, but it does seem to match the other information. The setting on battery meters would need to be an integration of the curve to get the average efficiency.

Looking at the chart, there would be no way to set a charge efficiency in a battery monitor or charger, unless you had exactly the same discharge amount on every cycle. This would make most of the SOC % readings you see very inaccurate on recharge.