Can I Charge Different Battery Chemistries With One Charger?

Question:

I hate asking you questions as I'm not even a customer, but if you move to Monterey CA I promise to give you business. That said, I watched all of your basic electrical videos and spent at least an hour researching this before I wrote this email. I'm going to get 2 Firefly Oasis AGMs for my house bank and use my current AGM for a starter battery. If I get a solenoid switch to charge the batteries from the alternator, I won't sleep at night knowing that the starter battery isn't getting the correct 3 stage charge (assuming a 3 stage regulator is installed) as I will set the voltage sensing wire to house bank. Maybe I misunderstood Nigel Calder, but I think that's what he recommends. Now, he talks about the Stirling Power Alternator to the battery charger and I'm leaning in this direction. My question, is this just over the top? Will the starter battery not be significantly harmed by bulk charging if it's in parallel to the house bank by solenoid? Is there another Blueseas product I am overlooking? This seems like such a common configuration, I can't imagine I'm the only one struggling with this. Another solution I came up with, I'm not sure is safe or effective, can I run the alternator to a distribution block, then to two independent 3 stage regulators for each bank? -Steve

Answer:

Ideally, all your AGM batteries would be Firefly. Notwithstanding, that is probably overkill and I wouldn’t worry too much about different AGM batteries in parallel while charging.  

Your parallel solenoid  (Bluesea ACR) is well-suited for this application and your engine AGM battery will see a similar, albeit slightly lower voltage (because of voltage drop) than the house AGM Firefly and much lower current than the house battery based on the size of engine battery (acceptance charge rate) and level of depletion (nearly full after starting engine) when engine is running. 

In my opinion, your system with ACR is good enough. If perfection is desired and money and time is endless, all Firefly and two alternators with external regulators, albeit total overkill and NOT worth it.