Originally Posted by pwb01
I have two batteries mounted on the A frame and actually tie 2 more in if I camp for a week. Anyway, I have a battery condition meter that I bought at an auto parts store that reads the battery condition in percent of charge. Equating the meter to the panel, I see around 20% to 25% charge on my meter when the lights on my panel are down to one light.
When I see 20% on my meter as I leave the campsite, (one light lit on my panel) I see 100% on my meter when I get home 1.5 hours later indicating a full charge. The three lights on my panel indicate full charge also.
Sorry so long getting back to. Was out camping and then work was busy.
OK now I see how you are seeing this. A few thoughts as to what may be going on. And possibly tricking the meters.
See this chart form Trojan Battery. go to bottom of page Trojan Battery Company
The Tank panel indicators are really not accurate
I think your year tank panel looks like this
If you look at the standing open circuit voltage on the Trojan chart lets see how that lines up with the tank panel.
The "Charge" indicator (all 4 red lights) lists the voltage can be 12.7 to 12.85. And Trojan says "open circuit voltage" can be 12.73 volts and is 100% state of charge (SOC). In this case the tank panel and the battery manufacture agree if the TT battery has been resting for 8 to 10 hours not being used much.
Now the "Good" indicator (3 red lights) lists the voltage can be 12.10 to 12.15. Trojan says 12.10 open circuit voltage is 50% SOC. Or the battery is 1/2 drained and the tank monitor calls it good. That is very miss leading.
The "Fair" indicator (2 red lights) lists the voltage can be 11.60 to 11.65. Trojan says 11.66 open circuit voltage is 20% SOC. For many, draining a battery down that far is not good for longevity of the battery, but the tank panel says fair.
The "Low" indicator (1 red light) Does not list how far the voltage is. Trojan says 11.51 is 10% or close to totally discharged.
I practice the 50% SOC charge guideline as the point where the battery needs to be recharged. This would be in the 12.10 volt area of a resting battery. By only draining down to 50% and then recharging, long term battery life can be extended if this is done on a routine basis.
OK now directly to what you are seeing. By any chance do you have a volt meter? It would be good to know what the % levels of your autoparts store meter considers what actual voltage.
Since the tank panel is one light lit and the auto part store says 20%, well the battery may be drained down to a real 20% SOC charge area that is around 11.66 volt area or maybe lower. It would be discharged pretty good. So we will say that number/reading may be fairly accurate.
Your truck charge line for sure must be working as you are getting some level of charge in the battery. The charge voltage I'm going to estimate is at least 13.6 volts maybe more. And if your doing this for an hour and a half you have put a level of charge in the battery. Just I do not see it being a full 100%. Here is why I say this.
When charging a battery at elevated voltage, the 13.6 volts is in that range, the battery voltage rises to meet it. They call this a surface charge. When you unhook a battery from a charger or your truck it may well read 13.5 or so. That surface charge is not the real state of charge voltage. It is artificially pumped up that high from the charging process. If you wait 8 to 10 hours and do not charge any further or use the battery that surface charge will be lost and the true'er voltage will show up to show the actual SOC. We call that the resting voltage or the open circuit voltage. You really cannot tell the SOC of a wet cell battery by voltage immediately after charging.
So, your tank panel would be seeing this high 13.6 volts and say 100%. Your autotester is seeing the same thing and tricking it to 100%.
If you have 2, group 24 batteries I'll guess they are 85 amp hour rated each. So that is 170 amp hours of total capacity. If they where drained to 20% that would leave 34 amp hours left. Or that you have to recharge 136 amps for 1 hour to get them back to 100%. There is no way I can see how the truck charge line can put 136 amps into the battery in one hour.
Even our on board converters with 3 stage chargers that boost to high voltage (14.4 vdc) only take the battery to 90% SOC and then it takes a really long time to jam that last 10% in as the battery resistance is then so high. Over night in most cases.
I "think" the elevated surface charge may be tricking your tester and the tank panel. To test this concept, next time you try this do the same thing as you are now. Test before, tow 1 1/2 hours then test and then after 10 hours of no battery use, test again and see what you get.
I'll "guess" you may be putting 10 amps through the charge line to those 2 batteries. In 1.5 hours that puts 15 amp hours of power back into where you started with a 20% charged battery bank (35 amp hours worth) which would come out around 29% SOC. (50 amp hours worth). Even if your putting 15 amps in the truck charge lines only about 33% SOC in 1.5 hours
I'm curious if this test comes out this way. I'm always open to learning more on this battery deal from what folks see. Some day if we end up at a M & G in the same CG lets try this test. I have a clamp on DC amp probe and we can measure what your truck charging line is putting in the battery. I'll have to try that myself on my own truck. This will help add actual data to the theory.
Hope this helps