Alternator and house batteries

I am replacing my AGM batteries in my motorhome 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. Will Prowse seems to be of the mind that they shouldn't be charged from 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 so the DC DC charger just limits the current from the alternator and makes it the correct voltage for the lithium batteries. 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?

Thanks

I just removed the isolator I had between the truck battery and the house battery and replaced it with a Victron Orion-TR smart DC-DC charger. It can sense when the engine is started and will start charging soon after the engine starts. Another nice feature I have used once when I let my house battery drain completely is power supply mode, where it takes power from the vehicle battery regardless of whether the engine is running.

A dc to dc converter is the way to go. Settings depend on the manufacturers specs. A good rule of thumb is to use no more than 1/3 of your alternators output for charging your batteries. That leaves 1/3 capacity for the vehicle and 1/3 for overhead. I have a 150 amp alt so I got a 50 amp charger. My DC to DC is connected directly to the vehicle charging system with nothing in betwee.

A problem I see with putting a DC DC charger between the Trombetta and the house batteries is if I needed the house batteries to boost low chassis batteries for start I couldn't. I assume current can't go in the reverse direction through the DC DC charger. So, bypassing the Trombetta with the DC DC charger and removing the wires that trigger the Trombetta to close to charge the house batteries from the alternator. This leaves the Trombetta available to close to use the house batteries as backup for the chassis batteries.

However, I read lithium batteries don't work very well for this purpose and it might be harmful to them if used to increase current for the starter. Is that correct? I'm back to eliminating the Trombetta altogether, in that case, and counting on the chassis batteries to always be able to start.

Thanks for the responses.

i installed this- https://www.amazon.com/gp/product/B00N4EF25W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1&fpw=alm

and this- https://www.amazon.com/gp/product/B07W84SS41/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

in my cargo van.works great.

neoflyer said:

A problem I see with putting a DC DC charger between the Trombetta and the house batteries is if I needed the house batteries to boost low chassis batteries for start I couldn't. I assume current can't go in the reverse direction through the DC DC charger. So, bypassing the Trombetta with the DC DC charger and removing the wires that trigger the Trombetta to close to charge the house batteries from the alternator. This leaves the Trombetta available to close to use the house batteries as backup for the chassis batteries.

However, I read lithium batteries don't work very well for this purpose and it might be harmful to them if used to increase current for the starter. Is that correct? I'm back to eliminating the Trombetta altogether, in that case, and counting on the chassis batteries to always be able to start.

Thanks for the responses.

Click to expand...

If you really wanted a momentary boost from the house batteries, just add a starter-type solenoid between the 2 banks and if the engine cranks too slow, you have a push button to 'jump' them together to help start it. My diesel pusher motorhome comes from the factory like this. Just push and hold the button, crank with the extra boost, then when it starts, let go of the button to break the 2 banks back apart.

Examples:

neoflyer said:

I am replacing my AGM batteries in my motorhome 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. Will Prowse seems to be of the mind that they shouldn't be charged from 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 so the DC DC charger just limits the current from the alternator and makes it the correct voltage for the lithium batteries. 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?

Thanks

Click to expand...

I pondered this for a while for my van. I was putting in a 240Ah Lifepo4 to replace a pair of golf cart batteries. I looked at several dc-dc chargers but decided to test the current alternator-switch-battery setup that I had. Based on limited use it has not been an issue charging directly from a 220 amp alternator (via the house batteries). The effect of a bit of resistance in the charging system adds up and results in the voltage at the batteries being significantly lower than at the alternator. This results in significantly lower charge rate than you might expect. At their current state of 10 Celsius and a 70% state of charge there are only 50 amps flowing, which is actually a bit disappointing! At lower states of charge and higher temperatures this will be higher, so this approach could still fail. However, so far I am happy that I did not get overly invested in hardware right up front. It would have been disappointing to be stuck with a 40 or 50 amp charge rate when part of the reason I switched to lithium was to get away from the limited charging rates of lead acid. For reference, the current van battery is connected to the lithium pack via 1 ga. wire, 3/8" steel bolts (through a wall), a novelty size switch, a shunt, and a fuse. This mix of good and bad connectors results in around 0.01 ohms of resistance! At 100 amps this would drop the alternators 14.4V to just 13.4V at the battery so the system is very self limiting. Of course, you lose any preprogrammed charging profile so you have to be comfortable with your own charge cutoff schemes. Floating the batteries would result in terrible things happening. I cut off the charge at a 3.6V max per cell, or when the total charge drops below 20 amps, whichever comes first. It is a new system however, with an inexperienced operator, so take that with a grain of salt.