Separate start house, different battery types

[mtompson]

Hello
I'm wondering about having two circuits, one for the engine one for the house. I have an AGM for the house, and a lead acid for the engine. My question is, do I share the common ground i.e. Engine block, or will that cause problems as I know I need to potentially keep my charge voltages apart for the two types of battery.?
So all positive cables are on their own circuit, but the negative cables can share the common bus.
Might be a waste of time, I'm just interested in people's thoughts. Many thanks.

[john61ct]

No, all DC circuits can (and usually should) share a common reference "ground".

The positive side is what gets combined / isolated as needed.

[john61ct]

You will likely find a setpoint that is acceptable to both types.

Otherwise a DCDC charger can be used

Otherwise focus on House's needs, ideally put all significant charge sources there directly.

Keeping Starter topped up is really trivial, and FLA much more tolerant of going out of spec.

When you next replace, matching units is a good strategy.

[S/V Illusion]

It is essential to ensure one common ground to avoid corrosion of underwater metals due to disparate ground potentials. There are lots of ways to ensure proper charge voltage for separate batteries.

[Mr O]

I use an echo charger for the start battery. It takes 12v from the house bank (AGM) to charge the start battery (FLA). Working fine that way for years.

[Chuck Hawley]

Originally Posted by john61ct

You will likely find a setpoint that is acceptable to both types.

Otherwise a DCDC charger can be used

Otherwise focus on House's needs, ideally put all significant charge sources there directly.

Keeping Starter topped up is really trivial, and FLA much more tolerant of going out of spec.

When you next replace, matching units is a good strategy.

Good advice! I think many of us get our panties in a bunch because we have heard that this battery should be charged at 14.3 volts and this other battery would be better at 14.5 volts. As this respondent points out, flooded atmospheric pressure batteries (not sealed) are really, really tolerant of higher voltages. They gas at some point, which is their way of dealing with higher voltages, which is actually find. Sealed batteries are a different story and due to their expense and they fact that they cannot be refilled with electrolyte, you do not want to exceed their recommended charge voltages. Regulate based on the sealed (agm) batteries and you should have no problem. And, if you can, equalize the flooded battery every once and a while with a small solar charger (unregulated) to improve its capacity.

Cheers,

Chuck

[mtompson]

Thanks all, great advice as always. I have the potential to just use the agm as it is designed to handle starts as well as house, but I liked the idea of having a cheaper lead acid just for starting and keeping a simple separate starting circuit, I have two alternators so one can keep him topped up

I’m not decided so just throwing the idea around, cheers

[bauer965]

I have 4 GCs for house and a Optima AGM for start. No problems whatsoever.
The settings on my Newmar charger are the same for FLA and AGM. The AGM has lasted over 10 years, and the GCs are over 5 years old.


Temperature compensation via a sensor to the charger is more important than dialing in the voltage precisely, as mentioned previously.

[Stu Jackson]

Originally Posted by mtompson

Thanks all, great advice as always. I have the potential to just use the agm as it is designed to handle starts as well as house, but I liked the idea of having a cheaper lead acid just for starting and keeping a simple separate starting circuit, I have two alternators so one can keep him topped up

I’m not decided so just throwing the idea around, cheers

The charge voltages for AGM and wet cells are very close. Not an issue. Want to see? Go to Balmar | Marine Charging Systems | Battery Monitors | Multi-Stage Voltage Regulators | High Power Alternators | Serpentine Pulley Conversions and download any regulator manual, like the MC-614. There is a chart with preprogrammed charging voltages. They are almost identical.