Chassis alternator charging deep cycle house batteries

[pgandw]

Originally Posted by BendOrLarry

One other point. The scenario I described above: we might go for close to two weeks without hooking up to shore power, with little generator usage, so my deep cycle batteries are heavily impacted by what ever the alternator is doing to them between stops.

This is the first I have seen of your usage scenario. I can understand your concerns, but from my knowledge of vehicles and deep cycle batteries, I believe they are misdirected.

You are right in that the alternator is not doing a great job of recharging your house batteries due to the presence of the chassis battery. But the alternator is NOT harming your deep cycle batteries in any way; it is simply not charging them efficiently.

Deep cycle batteries can only accept a maximum of about 20% of their amp hour capacity as a charge rate. 80AH deep cycle batteries can't take more than 20 amps in charging current. And 14.4V from the charger isn't even going to sustain 20 amps for very long. At 14.4V, the charge will rapidly taper to the 12-15 amp range. So even if the alternator maintained 14.4V, it will take 3 hours to take the battery from 50% to 90%. And it will take another 3+ hours at 13.7V to cram that last 10% into your deep cycle battery. So even with the best charger in the world, you need 5-6 hours to fully recharge a battery that is at 50%. Real world experience is 8+ hours to fully charge a 50% deep cycle battery, but around 3 hours to get it to 90% (assumes good wiring and no interference from the chassis battery).

Charging at a slower rate does not harm a deep cycle battery. If you recharge at maximum possible rates you run the risk of substantial energy going to other chemical processes besides moving sulfate from lead into the water. Things like heating the water into steam, and breaking water into hydrogen and oxygen are available to absorb any excess energy. Neither of those reactions are desirable - you want to minimize them while maximizing dissolving sulfate into water.

Leaving a deep cycle battery at 50% for days at a time can encourage harder forms of lead sulfate to form - harder forms that won't dissolve with application of charging voltage. This is called sulfation. Sulfation and accumulation of lead chunks at the bottom of the cell are the 2 primary causes of battery failure.

The ideal solution for your case is a second alternator set up to charge the house batteries, and leaving the existing alternator to take care of the starting battery. Sailboats with high-use house batteries often use a second alternator to minimize running time on the engine while maximizing recharge.

Without a second alternator, the best bet is to use the generator and/or solar panels to keep the house batteries between 50% and 90% while camping. If you can keep from repeatedly going below 50%, and get them fully charged between trips, you should get at least 5 year's life out of your deep cycle batteries.

Fred W

[BendOrLarry]

Originally Posted by pgandw

This is the first I have seen of your usage scenario. I can understand your concerns, but from my knowledge of vehicles and deep cycle batteries, I believe they are misdirected.

You are right in that the alternator is not doing a great job of recharging your house batteries due to the presence of the chassis battery. But the alternator is NOT harming your deep cycle batteries in any way; it is simply not charging them efficiently.

Deep cycle batteries can only accept a maximum of about 20% of their amp hour capacity as a charge rate. 80AH deep cycle batteries can't take more than 20 amps in charging current. And 14.4V from the charger isn't even going to sustain 20 amps for very long. At 14.4V, the charge will rapidly taper to the 12-15 amp range. So even if the alternator maintained 14.4V, it will take 3 hours to take the battery from 50% to 90%. And it will take another 3+ hours at 13.7V to cram that last 10% into your deep cycle battery. So even with the best charger in the world, you need 5-6 hours to fully recharge a battery that is at 50%. Real world experience is 8+ hours to fully charge a 50% deep cycle battery, but around 3 hours to get it to 90% (assumes good wiring and no interference from the chassis battery).

Charging at a slower rate does not harm a deep cycle battery. If you recharge at maximum possible rates you run the risk of substantial energy going to other chemical processes besides moving sulfate from lead into the water. Things like heating the water into steam, and breaking water into hydrogen and oxygen are available to absorb any excess energy. Neither of those reactions are desirable - you want to minimize them while maximizing dissolving sulfate into water.

Leaving a deep cycle battery at 50% for days at a time can encourage harder forms of lead sulfate to form - harder forms that won't dissolve with application of charging voltage. This is called sulfation. Sulfation and accumulation of lead chunks at the bottom of the cell are the 2 primary causes of battery failure.

The ideal solution for your case is a second alternator set up to charge the house batteries, and leaving the existing alternator to take care of the starting battery. Sailboats with high-use house batteries often use a second alternator to minimize running time on the engine while maximizing recharge.

Without a second alternator, the best bet is to use the generator and/or solar panels to keep the house batteries between 50% and 90% while camping. If you can keep from repeatedly going below 50%, and get them fully charged between trips, you should get at least 5 year's life out of your deep cycle batteries.

Fred W

I would be surprised if there are not quite a few people like us how actually travel from place to place without being in camp grounds most of the time.
I realize there are people who just go park their butts in a campground and plug in, but that's not us. We are going to have at least 600 watts of solar very soon and then this issue will be not an issue.
A 50% SOC is where I stop the discharge process for the reasons you mention, this is not my first rodeo, been owning RV's and camping for around 40 years.
I just suspect this issue is not so unique and was worth a chat.