Reality check - charging 2 batteries simultaneously

[N147JK]

Just want to make sure I understand this correctly..

1. I have a typical onboard, shore-powered battery charger.
2. My van has an old-school solenoid style battery isolator.
3. When the ignition switch is turned to either "on" or "acc", the solenoid closes and the chassis and house batteries are connected in series.

Question.. if I'm plugged in and the shore-powered charger is activated, and I turn the ignition switch to "acc", the onboard charger will automatically charge both batteries simultaneously. Is this correct?

Note: When I measure the voltage at each battery in this condition, I see over 13 volts at both batteries - it appears they are both receiving a charge.

Thank you electrical experts!

[RossDinan6]

Without an electrical schematic I could only make assumptions. That said, yes. Plugged in and key on I'd assume the charge rely is energized charging both the coach and chassis batteries.

But, why do you need to? Unless modified there should be little current draw from your chassis battery when sitting. Exactly like any other vehicle sitting in a parking lot. The engine ECM and dash audio will pull a few milliamps, a non issue unless sitting for a long time.

Ross

[N147JK]

Ross - thanks for the reply.

You're right, the chassis battery typically wouldn't need a charge. But it's good to know one can keep it fully charged by using the onboard charger, "just in case".

What got me thinking about this is the automatic solid-state isolators do this (I think). With the onboard charger on, they will charge the chassis battery, whether you want to or not. I like having the keyed solenoid - it gives one compete control over this function.

[eric1514]

I don't know why you'd want "complete control of this function" when you could install a Trik-l-Start and just have it automatically keep both battery banks charged as necessary.

[N147JK]

Originally Posted by eric1514

I don't know why you'd want "complete control of this function" when you could install a Trik-l-Start and just have it automatically keep both battery banks charged as necessary.

Yes that makes sense, but I don't want to buy another isolator if I don't need to. I just wanted to make sure I understood how the isolator I already have handles 2 batteries with the charger on.

However, with the charger offline (boondocking) the keyed solenoid isolator allows one to either combine the 2 batteries or not (if the chassis battery is good). I can see some scenarios where one might want to bring chassis power onto the house circuits. Will the SS isolators do that? Many of them say "batteries are isolated when discharging". What if one needs them combined while discharging? I'm guessing the costlier ones allow that.

[eric1514]

Originally Posted by N147JK

Yes that makes sense, but I don't want to buy another isolator if I don't need to. I just wanted to make sure I understood how the isolator I already have handles 2 batteries with the charger on.

However, with the charger offline (boondocking) the keyed solenoid isolator allows one to either combine the 2 batteries or not (if the chassis battery is good). I can see some scenarios where one might want to bring chassis power onto the house circuits. Will the SS isolators do that? Many of them say "batteries are isolated when discharging". What if one needs them combined while discharging? I'm guessing the costlier ones allow that.

To each there own, but I can't think of any time that I would want to discharge my chassis battery for the benefit of my house battery.

[markopolo]

Originally Posted by N147JK

Just want to make sure I understand this correctly..

1. I have a typical onboard, shore-powered battery charger.
2. My van has an old-school solenoid style battery isolator.
3. When the ignition switch is turned to either "on" or "acc", the solenoid closes and the chassis and house batteries are connected in series.......................

Just a note to keep terminology correct. When the solenoid closes the chassis and house batteries are connected in parallel not in series.

[N147JK]

Originally Posted by markopolo

Just a note to keep terminology correct. When the solenoid closes the chassis and house batteries are connected in parallel not in series.

Thank you for that - I was afraid I had that wrong.

[N147JK]

A follow-up question..

With the SS isolators (or solenoid isolator latched), if the charger is online, it will attempt to charge both batteries. But how does that work when one battery is fully charged but the other one is not? Can this lead to the charged battery receiving an "overcharge" and shorten its life? - Thanks

[booster]

Originally Posted by N147JK

A follow-up question..

With the SS isolators (or solenoid isolator latched), if the charger is online, it will charge both batteries. But how does that work when one battery is fully charged but the other one is not? Can this lead to the charged battery receiving an "overcharge" and shorten its life? - Thanks

It will depend on the charger and what stage it is in. Some might stay if float if that is where it was and some may not. If it is on full charge voltage in absorption the full battery will get overcharged which is not a good thing.

[N147JK]

Originally Posted by booster

It will depend on the charger and what stage it is in. Some might stay if float if that is where it was and some may not. If it is on full charge voltage in absorption the full battery will get overcharged which is not a good thing.

OK, interesting. Inversely, if I have a smart charger on 2 batteries, 1 depleted needing full charge voltage, the other charged and needing float, the charger may run on float. This will mean the depleted battery may never fully charge - not the desired outcome. Is this possible?

[booster]

Originally Posted by N147JK

OK, interesting. Inversely, if I have a smart charger on 2 batteries, 1 depleted needing full charge voltage, the other charged and needing float, the charger may run on float. This will mean the depleted battery may never fully charge - not the desired outcome. Is this possible?

It may be possible, but would depend on how the charger determines if a charge is needed. Many use voltage so it would depend on how high the charged battery could hold voltage when connected to discharged battery. If the discharged battery is quite discharged and they are of similar size, I don't think the voltage would hold high enough for most chargers to go directly to float.


All of this is why it is normally recommended that any battery bank use only identical in model, age, style and capacity and always charge and discharge together. You can't do that if the starting is battery in in the mix as you can be at risk of not being able to start the van.

[SteveJ]

I got to thinking....uh oh.

I understand and agree that matching house batteries is a desirable thing for discharging and charging.

As far as one battery being down and the other not, this occurs quite regularly to no apparent negative effect pretty much on every engine start up after one or more nights of dry/boondock camping. Not talking lithium, here. The joining of the batteries can include a starting battery to a discharged AGM battery, again to no apparent detriment.

Does one suppose that it is just a magical thing that it all seems to work out?

Not meant to be contradictory, just never thought this out before.

[booster]

Originally Posted by SteveJ

I got to thinking....uh oh.

I understand and agree that matching house batteries is a desirable thing for discharging and charging.

As far as one battery being down and the other not, this occurs quite regularly to no apparent negative effect pretty much on every engine start up after one or more nights of dry/boondock camping. Not talking lithium, here. The joining of the batteries can include a starting battery to a discharged AGM battery, again to no apparent detriment.

Does one suppose that it is just a magical thing that it all seems to work out?

Not meant to be contradictory, just never thought this out before.

It is not magical at all, just not a fall off the cliff issue that shows up immediately. Starting batteries tend to handle a bit of overcharge and undercharge better than true deep cycles do, but they also do fail sooner because of it. Undercharging of deep cycle lead acid batteries is a slow extended failure of the capacity continuously walking down until you don't have enough to run things long enough. Overcharging is generally a faster, but not immediate, type failure but is faster than undercharging failure and often will just stop working completely when they do fail.


This is exactly why many of us have manually controlled, or switchable manual/automatic, battery isolators. With them, you can easily prevent both over and under charging issues.


So the bottom line is that it works, but is not good for the batteries and shortens their lifespans if you chronically over or under charge them.

[N147JK]

Originally Posted by SteveJ

As far as one battery being down and the other not, this occurs quite regularly to no apparent negative effect pretty much on every engine start up after one or more nights of dry/boondock camping.

This is exactly what I'm wondering about. The engine side has an alternator and one regulator charging 2 batteries. Same on the house side - an onboard "smart charger" potentially charging 2 batteries. How does that work for each individual battery? Do the SS isolators route the charge as needed? Is there the potential for overcharge or undercharge? Or is that not an issue when batteries are wired and charged in parallel?

[booster]

Originally Posted by N147JK

This is exactly what I'm wondering about. The engine side has an alternator and one regulator charging 2 batteries. Same on the house side - an onboard "smart charger" potentially charging 2 batteries. How does that work for each individual battery? Do the SS isolators route the charge as needed? Is there the potential for overcharge or undercharge? Or is that not an issue when batteries are wired and charged in parallel?

See my reply in my post just before yours.

[SteveJ]

Booster, thankya, sir.

Our Roadtrek had a 4 stage converter charger in it that floated at 13.2(IIRC) after about a day. Our current Safari Trek has a multi(3, I think) stage inverter charger that also drops to about 13.2 or 3 after a day +/-.

While there are better chargers out there these have both apparently been good enough to keep the batteries in decent shape. I've been running the el cheapo "deep cycle" maintenance free batteries. The RT battery was a Wally deep cycle Maxx group 29. At 2.5 years when we sold it performance still seemed quite good. The Safari has 2 Interstate group 31 MHD batteries which are allegedly rated at 520 cycles, installed by previous owner. Usage is about 4 months/year of dry/boondock camping with solar charging to float virtually every day. Discharge/day runs from less than 15 to about 35%, occasionally more if it's really cold out, say in the 20-35* range. These batteries are just short of 2 years old and seem to still work well though I've not done a capacity test on them. I may do that upon return from our next trip that commences next week.

Have fun, y'all.

[booster]

Originally Posted by SteveJ

Booster, thankya, sir.

Our Roadtrek had a 4 stage converter charger in it that floated at 13.2(IIRC) after about a day. Our current Safari Trek has a multi(3, I think) stage inverter charger that also drops to about 13.2 or 3 after a day +/-.

While there are better chargers out there these have both apparently been good enough to keep the batteries in decent shape. I've been running the el cheapo "deep cycle" maintenance free batteries. The RT battery was a Wally deep cycle Maxx group 29. At 2.5 years when we sold it performance still seemed quite good. The Safari has 2 Interstate group 31 MHD batteries which are allegedly rated at 520 cycles, installed by previous owner. Usage is about 4 months/year of dry/boondock camping with solar charging to float virtually every day. Discharge/day runs from less than 15 to about 35%, occasionally more if it's really cold out, say in the 20-35* range. These batteries are just short of 2 years old and seem to still work well though I've not done a capacity test on them. I may do that upon return from our next trip that commences next week.

Have fun, y'all.

I like the 13.2 float better than the very common 13.5v we see a lot of, especially for long term storage. I think what is helping you a bunch for battery life is the very shallow discharges which will give you many more recharge cycles. I think you also have wet cells which are usually better at handling overcharging than AGM are as they will gas off the extra energy.



IMO, the most important thing is not the number of stages as much as how the charger the charger determines the batteries are full, which is a very difficult thing to do accurately and without a lot of high cost equipment. Even a lot of the expensive chargers do not do accurate charging. There are dozens of discussions on this forum from past years on accurate charging of lead acid batteries with plenty of background information and data.

[engnrsrule]

I believe my system (Roadtrek/Sprinter) is representative. 1. When driving, the engine alternator charges both the engine and chassis (coach) batteries. 2. When on shore (or generator) power, the 110v powers the battery charger which only charges the coach batteries. 3. When operating on coach battteries (boondocking) the load /drain is limited to the coach batteries. The engine battery should not see any significant drain unless you are running vehicle accessories.

Since coach batteries are (should be) deep cycle higher AH while engine battery is higher CCA, the coach battery charger may not be suitable for recharging the engine battery, even if such a connection were possible.

[RTRanger]

I’m happy with this solution:

https://www.amazon.com/gp/aw/d/B093BB3PCV

I removed the stock isolator and added Solar on the roof. Now when sitting around, the solar charges the lithium house batteries, when they’re full it switches to charge the engine battery. When driving, this unit will take a limited amount of power from the alternator and charge the house lithium batteries without the possibility of damaging the alternator. (Some lithium batteries can burn your alternator up since they have such a low internal resistance.)

I feel that batteries and alternators are too expensive and important not to have a setup that operates correctly.

The Bluetooth connection is a nice way to keep an eye on things!