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Forum Index : Electronics : 2 batteries, what to do.

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poida

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Joined: 02/02/2017
Location: Australia
Posts: 1419

Posted: 11:00am 14 Apr 2019

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Today, I find myself the proud owner of 4 x Vision HF12-890W in excellent condition.
open circuit voltage for all 4 is 13.27V +/= 0.01V
I did not plan to buy these, I sort of found that I had to buy them this weekend
for reasons.

I already have 8 x 12V 200AH REX-C Narada as the main battery. I'm quite happy with them actually. I run them with about 30-40% discharge cycle, no more. The specs
suggest this will give me 10 years or a bit more.

There are two solar arrays.
One is 3kW, North facing, with a Morningstar MPPT 60A controller.
The other is 3kW East facing, with a Morningstar 60A PWM controller.
Both controllers charge the Narada battery.
The inverter sources power from the Narada battery.

Problems:
How to charge the new battery.

Should I connect it in parallel with the existing battery and get on with life.

Is 2 batteries an advantage in some way?

Maybe make a new, self contained 48V DC battery/inverter system, to drive specific loads. I plan to get 2 of the smallest Mitsubishi inverter aircons this year
and I want to run them of solar as much as possible, then switch over to street.

What to do? Sell them? Keep them on float for 5 years then use them in a novel way?

Use it as a basis for setting up a friend's house with some standalone PV power, if he likes it he can then spend motza$$ and get BAE 2V gels or something, and get serious.

Make a PV car, the 4 batteries add up to 10kWh, 1/2 of the Nissan Leaf of a couple of years ago. No, don't want to deal with VicRoads. No, no,no.

Things I am aware of:

Different batteries will want and need different charge regimes.
Likely damage to battery service life will occur if I parallel these 2.

They were good and cheap. Not often I experience this in my life.

They are AGM valve regulated types, so the cycle life is not very big at all.
It's kind of "well what were you thinking, anyway?"

What's the mind of TheBackShed think I wonder?

wronger than a phone book full of wrong phone numbers

nickskethisniks
Guru

Joined: 17/10/2017
Location: Belgium
Posts: 458

Posted: 04:37pm 14 Apr 2019

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I would parallel them with your current battery. In my 48V I combine 180Ah SLA powersafe with 400Ah lifepo4 Valence batteries. I use a cheap active balance system to keep all 12v battery @ the same voltage.

I would also advice parallel mosfets in series between inverter and batteries and between solar and batteries. With an Arduino nano and mosfet driver you could easely program all sorts off protections.Edited by nickskethisniks 2019-04-16

renewableMark

Guru

Joined: 09/12/2017
Location: Australia
Posts: 1678

Posted: 09:55pm 14 Apr 2019

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Well you made the nano control boards to sync, why not make two inverters fed by their own batteries and sync the output of both.

Each battery bank can then have their own charge parameters.

Cheers Caveman Mark
Off grid eastern Melb

ltopower
Regular Member

Joined: 08/03/2019
Location: United Kingdom
Posts: 64

Posted: 10:57pm 14 Apr 2019

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The spec sheets dont seem to give any cycle life data appart from 3% self-discharge per month.

Parallel them up with your existing 48V setup, separating sets out only increases the cycle depth of the most used pack of cells....

ltopower
Regular Member

Joined: 08/03/2019
Location: United Kingdom
Posts: 64

Posted: 11:00pm 14 Apr 2019

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Here's a separate thought process...

Take the cost of the batteries and divide it by the kWh cycle life you think you can get out of the batteries.

Battery Cost / (kWh per cycle X Cycles)

This will give you a rough $/kWh cost equivalent to using them and is sometimes a good way to evaluate what to do.

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