Too many batteries and charge controllers!

[mikepepler]

A friend has just offered to sell me two 180Ah 12V traction batteries for £50 each. He used them in an electric boat which he's now sold. I know him well, and they've been well looked after, and are 5 years old. However, this brings me to my dilemma....

I have:
- 240W of PV, to be put on the roof in a couple of months. It's six panels, nominal voltages suitable for a 12V battery. I plan to wire them in three pairs, and bring the 24V lines into the house.

- a 15A MPPT charge controller. This can take an input up to 70V, and can charge either 12 or 24V batteries, no matter what the input is. I plan to use this with the panels.

- a Rolls 12V 80Ah deep cycle battery, which is only a year old and very lightly used, kept on float charge with some PV.

- lots of DC kit that runs off 12V, including a 180W pure sine inverter.

- separately, I have an 18W panel, 30A 12/24V charge controller (not MPPT) and a car battery. This runs a night light for our pet birds (very poor night vision!), and provides sockets for charging mobiles, etc. I plan to leave this system as it is, though the charge controller could be used as a spare if the other one failed.

So, my dilemma with the batteries I'm getting is do I:

A) Wire them in parallel, so keeping the system at 12V and being able to use all my kit as it is. The downside is that in peak sunlight I'll be missing a bit of the PV output (up to 30W), as the controller will hit the 15A limit. There's also the issue of my Rolls battery - I know it's possible to put it in parallel with the other two, but as the internal resistance will probably be different, one type will discharge more than the other, so reducing the life of it, and maybe all of them.

B) Wire them in series, allowing me to capture the full output of the PV and also have the MPPT be a bit more efficient, and have lower losses in the wiring. However, I then have to buy a 24-12V DC converter (£50-100 for a high capacity one), and am still left with the issue of what to do with the Rolls battery I have.

My thoughts on what I could do with the Rolls battery include:
- get rid of the car battery I have, and use the Rolls with the 18W panel, basically just keeping it ticking over as a spare while it runs the night light and a few other bits
- get a 12V charger with a DC input, and use that to keep it on float charge powered by my main PV system (whether that's 12 or 24V). These chargers are pricey though, being intended for use with RC cars. About £70 - and I already have a top quality mains charger that cost £100!
- leave the Rolls unused, and give it a top-up charge every few months. This could either by from the mains, or from the PV system via the inverter.

One other thought I've had is that for maximum redundancy of everything, maybe I should wire all the PV on the roof in parallel, keeping everything at 12 and interchangeable? This still leaves me deciding what to do with the Rolls battery though.

Any suggestions, or ideas I've not thought of?

[Bandidoz]

Can the 180W inverter run off 24V?

You could run two PV circuits - one of them containing 5 in series (so 60V into the charge controller that can tolerate 70V), with the other being a single panel, possibly charging your deep-cycle battery.

[mikepepler]

Can the 180W inverter run off 24V?

No, 12V only. When I bought it we didn't even own a roof to put the PV on, never mind having more batteries to play with...

[mikepepler]

I just read a suggestion somewhere that you can use a PWM charge controller to take power from the main system and use it to charge a second battery. Does that sound OK? I thought that controllers designed for PV were safe to connect to any other power source?

[mikepepler]

After some more thought, I'm thinking I should stick with 12V throughout, so that I have more redundancy in terms of batteries and controllers.

Any thoughts on the downsides of doing this?

I also realised that altough the MPPT controller is more efficient with a 24V system, that efficiency would be wiped out by losses in a 24v-12V converter. Thus 24V would only be a benefit if I got a 24V input inverter, which I won't for some time - could always rewire things if I do.

[adam2]

I would certainly stay with 12 volts unless very considerable expansion is planned.
A much greater choice of lamps and appliances are available for 12 volts.
12 volts is slightly safer, though with common sense the risks are minimal at 24 volts.

Except short term or in an emergency I would avoid parraleling batteries of different ages or makes.

I would keep the new 12 volt batteries as spares.
To keep them charged and in good condition, either uses a small dedcicated PV module, or a mains powered charger turned on manually or by a time switch once a week so as to avoid paying forever for the losses in the charger.

For a large system, 3 wire DC is worth considering as it gives both 12 volts for lighting and 24 volts for power.
Voltage drop is much reduced, the same size cable can carry 4 times the load (not 2 times) for the same % voltage drop.
Given enough charging capacity, 24 volts can power limited electric cooking.
Charge controllers and inverters are often slightly more efficient at 24 volts.
The drawback is extra complexity.

[mikepepler]

Thanks Adam, I think I am indeed going to stick with 12V, and keep whichever batteries aren't connected to the main system topped up.

Just out of interest, how do things work with a 3-wire system? Is that two 12V batteries in series for 24V, and taking the third wire off between the two of them to get 12V? Does that leave the batteries unevenly charged though?

[adam2]

Thanks Adam, I think I am indeed going to stick with 12V, and keep whichever batteries aren't connected to the main system topped up.

Just out of interest, how do things work with a 3-wire system? Is that two 12V batteries in series for 24V, and taking the third wire off between the two of them to get 12V? Does that leave the batteries unevenly charged though?

Will start a new thread re this