Measuring battery condition
Easiest way to measure battery level of charge?
Forum Discussion
KD4UPL wrote:
The right way to measure the state of charge of a battery is not a voltmeter. You need a battery meter. Also known as a state of charge meter, battery gauge, etc. It involves a shunt wired into the negative battery cable to measure the current into and out of the battery over time. You program the meter with your battery capacity and the fully charged parameters and it measures current in and out and calculates the percentage state of charge. Tri-Metric is a good one. Victron makes one. Outback calls theirs the Flexnet DC. Magnum calls their the ME-BMK. Link 10 used to be one but I'm not sure they are made anymore.
As you can tell from all the previous posts, measuring just the voltage is subject to many variations. I don't know very many people who, when camping, are willing to turn off the RV, disconnect their batteries, and wait 15 minutes just to find out the SOC. A nice SOC meter with a wall mounted display is the way to go.
I would agree with this. A voltmeter is a bit like the old joke about bikinis - what it reveals is interesting, what it conceals is essential.
On my previous truck I used a Tri-Metric and the battery monitor of the Blue Sky solar controller. Now I use the battery monitor of the Magnum inverter/charger.
With AGM batteries, I cannot do a hydrometer test and, as noted, in real life my batteries are always discharging - refrigerator, heat/fans, etc. And this does not count the induction stove/microwave or the espresso machine. ;) Simply not worth the effort to disconnect all loads for a half hour or more.
But this begs the real question - what are you trying to learn and why? A common answer is, "Well, I want to know if I have to recharge my batteries." Largely irrelevant as, at least with lead acid, the answer is always "Yes!" You want to recharge any time you can, every time you can, for as long as you can. So the real issue is to be sure that you have good chargers and that they are connected so that they come on whenever they can. One of two reasons that solar is so helpful.
I tend to watch the hour counter most of all - I blow off about 125Ah overnight and I always want to see it back to 0 as soon as possible.
N.B. With lead acid, that still means that you need hours of absorb charging.
Bottom line, most of us will be better off with some form of battery monitor. The SmartGauge is the simplest.
You can learn more about the SmartGuage and all manner of other things about lead acid battery charging here: SmartGauge The SmartGauge is sold in the US by Balmar.
