Charging House vs Starting batteries question (1 Viewer)

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I am at the point where I should be getting new starting batteries for my HDJ. My house is a relatively new AGM. Since they are all tied together (with a main switch and ACR) I threw the switch to 'all' and connected to my house. I could just tell my starting batteries were low so I was surprised that my charger went straight to float. I hooked the charger to my starter batteries and it is charging now. Why would that make a difference? I was thinking ACR, but if anything, I would have thought it would bypass the house and go to the low batteries. Now I assume at least my ACR is keeping my house from charging since it doesn't need it anyway. Why wouldn't I be able to charge all my low batteries from the same terminal?
 

MANUCHAO

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What is your total battery capacity (Ah). Make sure your charger is rated to handle your total battery capacity.
Charging different type of batteries is not ideal.......
I have mine connected with an ACR as well, but use a two bank ( 20 amp total) charger for both batteries... Both batteries being AGMs
 
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Don’t remember the Ah for my starting but they are typical flooded and spec. Pretty sure they are 880 CCA. My AGM is 100Ah. I am aware of the different battery types but how would my charger know that from post to post?
 

e9999

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I don't know what "charger rated to handle Ah capacity" means actually. A (moderately smart) charger typically has a current limit based on its internals. If the battery is not yet charged- that's what you gonna get. It doesn't matter whether it's a 100Ah or 200Ah. It will just take longer to charge the battery in the latter case. Well, at least, that's what I've observed.

As to internal resistance, if you had 2 batteries with different resistances connected in parallel when charging, there may be a difference in internal current and charging speed but the voltage will remain the same, of course. And the currents adapt themselves to that voltage. Charging should not be a problem until you get to the point where the voltage needs to be higher to achieve full charge for one over the other, as in AGM vs FLA. You'd want to set the max voltage (or use the corresponding charger) to the lower one to avoid overcharging of the lower one, but then the one needing the higher voltage would not be fully charged. I don't think that's a big problem though. Myself, I would probably charge them separately if I had a choice, just cuz I'm particular and would want both to be charged optimally, but I doubt things would blow up if you are reasonably careful when choosing the hardware. (The above hypothetical considerations assuming no weird control switching issues, of course.)
 
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I really want to separate the charging of my flooded vs my AGM. But right now I was just curious about actual charging one over the other. I think resistance is the key. But I only think that works if the battery needs charging. If the battery is fully charged then the charge should float to the lower voltage (?)
 

e9999

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What do you mean about resistance?
On the FLAs, you'd likely want to do bulk charging first to around 14.4, then some absorption at that level then float at around 13.7 or so. My AGM says to charge it first to 14.6-14.8 so a bit higher and then 13.6-13.8 for standby so similar float level. I would think that if you'd charge the AGM to 14.4 together with the FLA (in parallel) and then float both at 13.7 you'd likely be fine. In fact, I have done exactly that on my bench with no visible ill effects (yet?). I imagine it's possible that long term (years?) of not ever going to 14.6+ might affect things adversely for the AGM but also kinda assume it won't be dramatic. Of course it's best to just call the manufacturer of the AGM and ask, better than some random anonymous internet shadowy figures... :)
Ultimately, though, it's the charger that will determine what's happening (constant voltage, or smart cycling or...) If you have a modern programmable charger you can decide on what voltages you'll get and easily switch between settings if charging the batteries separately. Of course if you're talking alternator and with ACRs etc in the circuit, it's a completely different ballgame.
 

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