RV solar and alternator question

[sdbuck]

My RV's (VW Rialta) house batteries are connected to a solenoid that connects to the vehicle's alternator. The solenoid enables the house batteries to be charged while driving, but also disconnects when the vehicle's off so the house batteries don't drain the engine's battery.

My charge controller is the Tracer-3215RN. It's MPPT and has 4 charging stages (I think).

QUESTION:

Should I disconnect the house batteries from the solenoid or alternator so that they're only charging from my solar panels? I feel like the alternator is disrupting the charging phases of the MPPT controller and giving it false voltage readings (while I'm driving and for a little while after, the batteries are at 13.5-14V, even though they're actually between 12-13.5).

The only advantage I see to keeping them connected to the alternator is when the batteries are low, the alternator can probably initially charge the batteries faster than my panels because it's much higher amperage. Also, if my batteries are low at night and I'm driving somewhere, I can charge the batteries a little more. Maybe I should install a switch?

ALSO, my AGM batteries are at 13.5-13.7 and I'll disconnect everything that would drain them. When I come back a few hours later, they're around 12.5V. Is this because they aren't being charged properly (alternator is getting in the way of the MPPT function and not fully charging them) or because my batteries are shot? Batteries are 6 months old but were constantly under charged before I got the solar panels - they were below 10.5 or 10V once.

[Mike90250]

in a word NO, the alternator and solenoid will not confuse the MPPT controller.
5 minutes of running the engine will put a days worth of solar charge into the battery. So it's likely that

a) the battery is getting old/tired/ruined from being older than 3 years, or not enough sun to recharge it.

b)

Batteries are 6 months old but were constantly under charged before I got the solar panels - they were below 10.5 or 10V once.

That will do them in.


Make sure the solar parts are working well before investing in the new batteries.

[Sunking]

Do not disconnect the alternator/solenoid circuit. The alternator can do more in 15 to 30 minutes than the panels can do in a week. If anything get rid of the panels as they are not contributing much of anything.

[Leehamrick]

Boats have two batteries one is for starting the boat and one is for using the running lights at night
when your just floating around at night parting or fishing or what ever. And they work this way.
I'm not sure but I think some how it does it by it's self. I don't know if a switch is used hooked to the
ignition switch or if a diode is used. But yes I would think this would work fine.

[sdbuck]

Thanks guys! Your answers are the same as I got when I called an RV Solar Installer...the MPPT controller can figure it out...best to keep it all connected.

My batteries are 82AH @ 8hr Rate AGM 12V (two of them). Float charge is 13.5 +/- .06 V. I'm still not exactly sure what 100% fully charged Voltage should be. I've looked at charts online and some say 12.7, others 12.6 or even 12.5 for AGM.

I think my batteries actually go down to about 12.2V without anything draining them (a rear view camera is the only thing I leave on all the time, can't imagine that would drain .3V-.4V off a 164AH battery bank...)

I guess I'll bring the batteries to an auto store to have them tested. I should have 984 watts to play with before draining them past 50% [(82AH x 2 batteries x 12 volts)/2 for 50%) and they seem to be closer to 12.1 after just ~300 watts which doesn't make sense. Unless they're shot. Even then, they function pretty well, just doesn't seem to be as good as they should be if my math is correct.

[Sunking]

Well for one you need to get your math straight. Batteries are measured in Amp Hours or Watt Hours. The number you want to work with is WATT HOURS. To find a battery Watt Hour = Battery Voltage x Amp Hours.

Watt Hours = Watts x Hours.

So you have 2 12 volt 85 AH batteries right? That is a total capacity of 2 x 12 x 85 = 2040 watt hours of which only about 1000 watt hours are usable to 50% DOD. So your 300 watts means nothing. You want to know watt hours.

[Wy_White_Wolf]

...ALSO, my AGM batteries are at 13.5-13.7 and I'll disconnect everything that would drain them. When I come back a few hours later, they're around 12.5V....

Normal. The 13+ readings are surface voltage readings and therefore not an accurate reading. Batteries shouls set for at least 3 hours without load or being charged to get an accurate voltage reading.

WWW

[SunEagle]

Thanks guys! Your answers are the same as I got when I called an RV Solar Installer...the MPPT controller can figure it out...best to keep it all connected.

My batteries are 82AH @ 8hr Rate AGM 12V (two of them). Float charge is 13.5 +/- .06 V. I'm still not exactly sure what 100% fully charged Voltage should be. I've looked at charts online and some say 12.7, others 12.6 or even 12.5 for AGM.

I think my batteries actually go down to about 12.2V without anything draining them (a rear view camera is the only thing I leave on all the time, can't imagine that would drain .3V-.4V off a 164AH battery bank...)

I guess I'll bring the batteries to an auto store to have them tested. I should have 984 watts to play with before draining them past 50% [(82AH x 2 batteries x 12 volts)/2 for 50%) and they seem to be closer to 12.1 after just ~300 watts which doesn't make sense. Unless they're shot. Even then, they function pretty well, just doesn't seem to be as good as they should be if my math is correct.

You may be having some issue due to the batteries are wired in parallel. Even with just two there is a chance of unequal charge and discharging which may have weaken one of them.

[CraziFuzzy]

You may be having some issue due to the batteries are wired in parallel. Even with just two there is a chance of unequal charge and discharging which may have weaken one of them.

When you inevitably replace the batteries, get away from wiring 2 batteries in parallel. Doing so greatly shortens life, as one of them starting to die will destroy the life of the otherwise good one. You will always be better off wiring batteries in series. In your case, best/most cost effective/most common option is 2 6V golf cart batteries in series. Keep the solenoid and alternator in there, to get better charging when available.

[SimonHiluxGalaxy]

I need to turn this question around - if my solar panel is lifting the voltage of the battery, but not pushing much amps, then wont the car alternator see the higher voltage, and therefore consider the battery to be charged and not send it any current? IE the car wont charge the battery with the solar system connected???

[Mike90250]

I would expect the alternator to seldom throttle back from effects of the solar charging.

[Wy_White_Wolf]

I need to turn this question around - if my solar panel is lifting the voltage of the battery, but not pushing much amps,...

How would that happen if the battery isn't fully charged?

WWW

[SimonHiluxGalaxy]

All batteries will only charge by lifting the charging voltage higher than the battery. Due to the hysteresis of the battery, the voltage reading is higher than the voltage after the charge is removed. Image a river flowing into another river. No water will move if they are both at the same level. Water only flows from one to the other if the river is higher. Like wise with a battery - any battery needs a higher voltage presented to it in order to charge, and the battery voltage will be higher while the charging current is applied. Eg to charge a 12volt battery, the alternator usually presents 13.47 volts to the battery. Now if the solar panel is already raising the voltage, then the alternator may think the battery is charged and not try to charge it also, even though the alternator has much more current available to charge the battery, but it wont deliver it when it sees the battery voltage is higher. It will think that is the resting voltage . That is my theory anyway. Comments?

[Wy_White_Wolf]

If your batteries are being fully charge by the alternator at 13.47 volts then they are being fully charged by solar at 13.47 volts. Actually voltage will also be related to the amperage of the supplied charge. Higher the amperage the higher the difference is resting and surface (charging) voltage. So with that in mind a 13.47V charge from the solar will mean your batteries are charged to a higher SOC than the alternator would do at 13.47V.

If your solar shuts down the charging from the alternator that means your batteries are charged to or above the ability of the alternator to charge them.

WWW

[Sunking]

I need to turn this question around - if my solar panel is lifting the voltage of the battery, but not pushing much amps

That would mean your batteries are:

1, Charged up already.

or

2. Dead and need replaced.

95% of all battery failures occur like this. You connect them to the charger, and in a few minutes or seconds the charger is fooled into think the batteries are fully charged up. You connect something to the batteries, turn it on, and NOTHING but a Brown or Black Out from under voltage. It means your batteries have sulfated and has very high resistance. As soon as you apply a charge current, the voltage shoots up and fools your charger into thinking the batteries are charged up. You battery will no longer take or give a charge.

With electricity, the source in a higher energy state provides the power. In the case of an RV with both an alternator and solar, the alternator is the higher energy source when the engine is running.

[SimonHiluxGalaxy]

Sunking - my batteries are brand new. 180AH 12V. Im trying to run a 7amp fridge off them and the 190w panel. I expect to be able to suck at least 90ah from the battery after sundown, but the battery goes flat (the regulator disconnects it at 10.5v) about 2 hours after sundown. So I must conclude the battery was not fully charged by solar. Ok fair enough BUT if I have been driving all the day, the result is the same. So I conclude the alternator is not charging the battery. Why? I assume because the solar panel lifts the battery voltage up enough to confuse the alternator not to charge it.

Wy_White_Wolf - yes if we assume both solar and alternator charges the battery by presenting 13.47 volts to it, then if the solar is doing that, but only pushing small amps, then the alternator wont push amps, even though it has lots more available.
I am assuming that with charging happening, the battery voltage reads 13.47 volts, BUT it is not necessarily fully charged. Am I correct?