Thanks for the input everyone.
I have the system charging on the solar now and will start the voltage tests as soon as it is "full".
Half down would be 77.5ah. 300w / say 14v would be 21a. Ignoring all the rest of the math and just roughing it, you're looking at 4 hours or so to bulk charge and maybe double that for the absorb.
If that battery is halfway down, I suppose it's possible that you'll get it back to 100% in a single day with a 300w solar array...but I have my doubts. It would be a lot more possible if you could program the voltage set points of the charge controller so it pumps up the voltage to say 14.8v bulk, 14.4v absorb (good numbers for most AGM/flooded but way too high for a GEL). If it's pre-set to the very common settings of 14.4v/14.2v, then it'll take longer.
I am using a monitor attached to my charge controller to determine that the battery is discharging to 50% in three hours.
Well, I could drive a truck through what is wrong with that...
What kind of monitor attaches to a Renogy charge controller? Morningstar has a display unit that attaches to their charge controllers, but exactly WTH are you talking about when you say monitor attached to your charge controller?
You say you tested the fridge load and it draws <5a. How did you test it? Clamp on ammeter?
If so, then fine. Now do am amp load test on the hot wire coming off the house battery. What's the load? It's gonna have to be around 25a or more to pull that battery down 50% in 3 hours. (That is...
IF that battery actually started out full in the first place, which is a BIG IF.)
Next...WHERE is the charge controller connected? If it's direct to the battery, okay, but if it's way down at the other end of the vehicle, on the same wire that is feeding the fridge, then the fridge could be pulling down the voltage (voltage drop under load) on that one circuit, and since the charge controller reads a dropped voltage, it is fooled into thinking the battery voltage is low - even if it isn't.
The starter is a flooded cell and the house battery is an agm. I am getting the impression after reading your post that charging the house through the starter battery is not good in my situation.
That's not how it works. The current to charge the house battery doesn't go
through the engine battery - it goes
around the engine battery.
The voltage regulator turns the alternator on and off so that the alternator supplies enough amperage to hold the voltage of the "12v bus" at whatever voltage the voltage regulator is set to - let's say 14.5v (depends on your vehicle). The batteries will suck power from the bus as best they can and by doing that will draw down the bus voltage, which will cause the voltage regulator to keep the alternator turned on. The alternator will supply however much power is being sucked out of the bus (by batteries and other loads like lights/radio/ac).
How much power will the batteries suck out of the bus? Depends on their state of charge. The engine battery will probably be at 99+% so won't be drawing much - maybe an amp, maybe less. Starting the engine uses around 1/5 of one amp*hour (or less), so it doesn't take much to top off the cranking battery.
The house battery will draw however much it can based on its internal resistance. At 50% the internal resistance is about as low as it gets. The resistance goes up at both ends of the graph - higher as the battery gets closer to dead, higher as the battery gets closer to full.
So, the charging setup that you linked to (Evil Dave's tutorial of how to rig a "split-charge relay") works fine -
no matter if the batteries are the same type or not -as long as the batteries can handle the 14.5v (or whatever voltage) your voltage regulator is set to (and the only ones that can't are usually GEL type), it's all good.
[Well...it's all good except for the fact that a voltage regulated alternator is basically a crappy constant voltage charging system that can take forever (24-36 hours) to fully top off a big battery - unless you have a super-duper modern computer controlled charging system, in which case, it might actually do a pretty decent job in a reasonable amount of time.]
Would replacing the starter battery with an agm solve the issue, or is the problem that my stock alternator wont provide the proper charging profile for an agm?
First, you have to prove that there actually IS an issue. But no, changing battery type isn't going to matter since AGM and flooded both use basically the same charge profiles. Typically AGM likes a higher voltage, which is actually BETTER (healthier) for a flooded battery - but will cause it to use a bit more water and need to be topped off with water more frequently.
The charge controller does have a setting for a sealed battery and the guy at renogy told me it would play nice with this specific battery when i purchased it.
Gotta RTFM to know exactly what the "sealed battery" setting actually does. It might just disable the "equalize" function, or it might reset the charge voltage set points to lower values (which would be sucky).
I'm thinking possibly there is something hinky about that on/off switch you added to the split-charge relay setup. Is it just a kill switch? I.e., you can turn off the relay when the engine is running, OR can it be used to connect the two batteries even when the engine
isn't running.
