Two batteries in rotation

[Matty Van Halen]

Two batteries in rotation   I’ve changed my battery set up.  I now have two deep cycle batteries. When I’m using battery number one, I am charging battery number two. When battery number one gets low, I switch over. This has worked seamlessly For me.   

 I mostly using my solar panels to charge but I also have a trickle charger that I plug in to the built-in inverter in the dash of the van.

So far this is provided in endless power for me. And peace of mind knowing I have a battery ready to go when the other one gets low

 

[John61CT]

Not optimal.

Greater total AH capacity and less voltage sage running them as one bank. See Peukert.

Shallower cycling also brings much longer life.

http://www.smartgauge.co.uk/splitting.html

 

[MrNoodly]

So you have one battery charging and one not? You realize, don't you, that with a standard setup both batteries would be charging as you use them? Your way is like drinking from one glass while the other is filling instead of drinking from a glass twice as large while it's being refilled.

 

[RVTravel]

It should work just fine when you are a low electric user.

 

[highdesertranger]

I don't see the advantage. please explain. highdesertranger

 

[Matty Van Halen]

It should work just fine when you are a low electric user.

Exactly. Hymer class B RVs have this very option.

 

[B and C]

I have the same question as HDR. The advantage over parallel is???

The Hymer brochure I downloaded showed 2x6V batteries.

 

[MrNoodly]

I don't see the advantage.  please explain.  highdesertranger

Maybe it's a peace-of-mind thing.

 

[John61CT]

Split banks used to be the conventional wisdom decades ago on boats.

Long myth-busted, but traditions die hard.

 

[Matty Van Halen]

The only advantage is if you accidentally leave something on it drains the battery down you just killing the one battery and you got another ready to go but other than that there is no advantage so yes it’s just a peace of mind thing for me. What works for one may not work for another

 

[Weight]

You are charging with charge maintainer that is fed from the battery?

 

[John61CT]

The only advantage is if you accidentally leave something on it drains the battery down you just killing the one battery

LVD cutoff is better for that.

But sure, your rig your choice, just warning of the issues.

 

[Ticklebellly]

Two batteries in rotation 

Currently doing this myself.   Li iron batteries.   Mostly playing with end charge volts number and establishing low volts cutout.   Also answering the question about the value of LiFePo4 technology for low weight and a Charge-and-Use way to use a quite flat discharge curve, all the way to the end.

Second advantage is that I am often using a portable panel to charge the second battery, in the sun, while the van is parked in the shade.

 

[John61CT]

And since voltage sag is less and Peukert coefficient is very close to 1.0 for LFP, you aren't losing much.

With lead you lose a lot.

 

[Spaceman Spiff]

Glad this is working for you.  But for others that are thinking about switching between batteries, here are a couple of things to consider for lead acid batteries:

1. You will discharge your batteries twice as deeply as they would if coupled together in parallel.  A deeper discharge shortens their life.  Both of my batteries are usually charged to 100% every day.
2. Any excess solar output is wasted, when the off-line battery is charged the excess is lost.
3. You are constantly swapping batteries, either by a complex switching mechanism or manually unhooking and switching them.
4. Your working battery is doing all the work, your solar is not running your system during the day.  My solar runs my refrigerator (my only big load) plus charges my batteries when the sun is shining.  My batteries are usually at 100% by 1 PM and the batteries usually don't start discharging until the solar input falls off (~4 PM).

I don't know enough about lithium battery performance to comment.

 

[Ticklebellly]

1.   A deeper discharge shortens their life. 

I do not hold that to be true of Deep Cycle technology and design.   Perhaps true if you consider all discharge/charge cycles to be the same.   Deep cycle batteries are designed to be deeply discharged.   Now we get into discussions about what exactly is "life".   Cycles?   A discharge event down to 20% is not the same as a discharge down to 40%, especially if the energy given up during a discharge event is considered.

If I have paid for a deep cycle capability, I want to use it.   Anxiety about not discharging past some arbitrary figure of 50% seems nugatory effort.   Gets even more murky when I see so much discussion on how to establish what the SOC is of any specific battery.   In my LiFePo4 project, I am finding it very easy to set a top voltage number as the top I wish to get to and set a Low voltage cutoff as the lowest I want to go, all the while I am doing some capacity measurements of the cells making up the battery.

 

[John61CT]

No anxiety required, feelings do not change the fact

> A deeper discharge shortens their life.

Obviously make your choice, either way is fine.

I prefer to build in a decent reserve myself

 

[tx2sturgis]

 When battery number one gets low, I switch over. 

What does the solar charger do when you switch over? How are you making the 'switch'? 'make-before-break' or 'break-before-make'? 

I can imagine some scenarios that would involve arcing, or, batteries that are slightly different voltages, when connected, might cause some high current surges.

... peace of mind knowing I have a battery ready to go when the other one gets low

The battery that is in use wont get as low if it has another one in parallel with it, assuming the same loads are in use. 

I can see some advantages, but for the average user, the 'cons' might outweigh the 'pros'.

A better solution to redundancy (in my opinion) is to have seperate systems, where two or more batteries are charged by seperate panels and controllers, and the 'loads' are moved from one bank to the other as needed or desired. 

This is how my systems are set up.

 

[Matty Van Halen]

I might give that a try if my power needs increase but so far it’s ideal for my situation.

After re-reading my original post I guess I should’ve been much more clear. When I say “low” I mean 12.6 Volts. So I’m not discharging them very deeply. I use AGM.

I use SAE plugs. I hook up to the battery first then to the solar panel. I’ve never noticed any problems

Now that the weather has gotten cooler I don’t even need auxiliary batteries. All I was using them for was to run a fan and a few LED lights. I ran the fan during the heat of the day and all night while I slept. I charge all my devices ( laptop and iPhone )from the two 12 V ports built into the dash of the van and I also have a 120 V inverter built into the dash.

 

[MrNoodly]

Being a big believer in convenience (a.k.a. laziness) the idea of switching connections twice a day, and making sure not to forget, and always needing to be around to make the switch, just seems like too much work -- unless there's some kind of automated device to handle it.