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Alternator and house batteries


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I am replacing my AGM batteries with lithium soon.  I have read there is some concern about charging the lithiums from the alternator since they can draw so much current it is harmful to the alternator.  I have some ideas for how to deal with that I will mention later.  Right now I need an education on how my charging system currently works. 

My '05 HR Ambassador has a Trombetta relay that opens and closes to allow the house batteries to charge from the alternator when the chassis batteries are fully charged, as I understand.  I have been told the relay closes if the alternator is supplying current and the chassis batteries reach 13.6v charge and the house batteries are 13.2v or less.  Is this correct? If so what sends the signal to close the Trombetta? 

I'm thinking of putting a DC DC charger that has a lithium schedule in the system.  I could put it in the line between the Trombetta and the house batteries.  That would keep the house batteries from being able to be used to connect to the chassis batteries for starting if they were low. That brings me back to the question of what signals the Trombetta to close.  Would it happen at the correct voltage for lithium batteries.  Is 13.2v, if that is the number, good for lithium or should it be something else?  I guess I could put the DC DC charger directly between the alternator and house batteries, bypassing the Trombetta altogether and charging the house batteries continuously.

Can someone help my work my way through this?


Edited by saflyer
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I don't have a Trombetta in my arrangement, but I can comment in general...

There's nothing specific to the battery technology in that 13.2v threshold. If the chassis-side of the system is above 13v, the system knows that the engine is running and the chassis batteries are charging; and therefore it should connect the two systems to allow alternator current to flow over to the house side.

I am also concerned about how much current the lithium batteries would draw from the alternator. But I'm not sure that concern is entirely legitimate. Think about the all-electric coaches that have eight golf cart batteries. Four GC batteries like I have will easily take >100a when they are low. So eight AGMs would draw far more than the alternator can produce. But that's apparently not causing rampant alternator failures?

Battle-Born is apparently not that concerned about it. From the BIM product page:

"The LiFePO4 Battery Isolation Manager (BIM) monitors voltage and connects batteries when needed. Under normal charging conditions, the BIM will connect for 15 minutes every 35 minutes. That means that the BIM will connect for 15 minutes, disconnect for 20 minutes, and repeat this cycle until the coach battery is charged.

If the coach battery resting voltage exceeds 13.4 V then the BIM will disconnect. A resting voltage greater than 13.4 V indicates a fully charged battery. Note that “resting voltage” means that no current is flowing to the coach battery.

The BIM will disconnect if the alternator voltage exceeds 14.4 V. This protects the coach battery from over charging. The LiFePO4 Battery Isolation Manager will disconnect if the voltage difference between the alternator and the coach battery is less than 0.1 V. If the voltage difference is too low, then there is a negligible charging current, and no need to connect to the coach battery.

The BIM will disconnect if the alternator voltage drops below 13.3 V. If the alternator voltage is too low, then it cannot adequately charge the coach battery, so there is no reason to connect."

Personally, I'm leaning toward removal of the original battery isolator and installing a small dedicated battery charger for the chassis batt. And then later maybe install either a DC-DC charger or the battleborn BIM in place of the existing battery isolator. 






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I'm thinking of doing what you suggest in the last paragraph.  Keep it simple for now with a DC DC charger and disconnect the Trombetta for now. We don't boondock for very long so the chassis batteries should hold up between starts.



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