Idiot's guide to a self build battery

[Jinx]

I have acquired a few cells recently and would like to put them to use.

Can anyone confirm that if I series up 16 cells (few strings in parallel) then I can get a small Victron Multiplus 2, GX Cerbo and be up and running?

I have an existing grid-tie PV system that I would like to keep but separate as possible so just AC coupled. Can the Victron detect (energy meter required?) when I have excess PV and charge the battery, or whilst at deficit, power the house? I’m only thinking of going 800VA currently so I’m guessing if I stick the kettle on the Victron will happily supply max rated power while the grid tops it up?

Here’s the cells I have got, relatively small at approx 70Ah but I have quite a few…

[Stinsy]

I have acquired a few cells recently and would like to put them to use.

Can anyone confirm that if I series up 16 cells (few strings in parallel) then I can get a small Victron Multiplus 2, GX Cerbo and be up and running?

I have an existing grid-tie PV system that I would like to keep but separate as possible so just AC coupled. Can the Victron detect (energy meter required?) when I have excess PV and charge the battery, or whilst at deficit, power the house? I’m only thinking of going 800VA currently so I’m guessing if I stick the kettle on the Victron will happily supply max rated power while the grid tops it up?

Here’s the cells I have got, relatively small at approx 70Ah but I have quite a few…

Yes you can is the short answer.

Why the Victron? High-priced premium-grade kit with loads of features you won’t use. Any “storage inverter” or “battery inverter” or “hybrid inverter” will do the job. You just need a CT to measure current flows on your main tails so it knows how fast to charge/discharge. You also need a “balancer”.

Are those LiFePO4 cells?

You have a choice, you can parallel up strings of maybe 16x cells or you can put them all in one string and get a so-called “High Voltage” (HV) inverter.

[Jinx]

Yes you can is the short answer.

Why the Victron? High-priced premium-grade kit with loads of features you won’t use. Any “storage inverter” or “battery inverter” or “hybrid inverter” will do the job. You just need a CT to measure current flows on your main tails so it knows how fast to charge/discharge. You also need a “balancer”.

Are those LiFePO4 cells?

You have a choice, you can parallel up strings of maybe 16x cells or you can put them all in one string and get a so-called “High Voltage” (HV) inverter.

Thanks Stinsy, the Victron was chosen as looked the cheapest option if I stuck to lower power (500-800W). I checked some hybrid inverters on eBay but even second hand these were more money, although granted much more powerful.

I’m happy to spend less if it will do what I need. One issue is the location for the battery will be some distance (probably a separate outbuilding) from the CU and meter in the house.

Yes they are LiFePO4, from a commercial storage battery but they are currently configured as 22 series but 2 on each pack are possibly unusable due to terminal damage.

[marshman]

Beware of getting an inverter that is "just big enough capacity". You WILL regret it sooner or later (most likely sooner) when you decide you NEED more batteries and a bigger system - and you will.

My advice would be to get a 3.6kW 48V hybrid inverter or just an AC coupled inverter/charger if you have no PV to feed in to it. Form a 2p16s pack - or possibly a 2p17s pack (a member on here was going to try that) from the cells you have - I am assuming you have 44 cells with 4 damaged ones. Then you can add to it later when you see for yourself how good it is.

[Stinsy]

Yes you can is the short answer.

Why the Victron? High-priced premium-grade kit with loads of features you won’t use. Any “storage inverter” or “battery inverter” or “hybrid inverter” will do the job. You just need a CT to measure current flows on your main tails so it knows how fast to charge/discharge. You also need a “balancer”.

Are those LiFePO4 cells?

You have a choice, you can parallel up strings of maybe 16x cells or you can put them all in one string and get a so-called “High Voltage” (HV) inverter.

Thanks Stinsy, the Victron was chosen as looked the cheapest option if I stuck to lower power (500-800W). I checked some hybrid inverters on eBay but even second hand these were more money, although granted much more powerful.

I’m happy to spend less if it will do what I need. One issue is the location for the battery will be some distance (probably a separate outbuilding) from the CU and meter in the house.

Yes they are LiFePO4, from a commercial storage battery but they are currently configured as 22 series but 2 on each pack are possibly unusable due to terminal damage.

I’d go for a 3.6kW inverter.

Be careful of cable size if your batteries/inverter are a long way from the CU. If your grid-voltage is already on the high side then your inverter could struggle to export at full power without hitting the max allowable voltage.

For example: if you do the standard sums then 3.6kW over 2.5mm² with a 30m run gives a 7.5V drop, which is perfectly acceptable in normal conditions but if the inverter is exporting the 3.6kW the voltage will increase V by 7.5V. Therefore 6mm² might be the sensible choice even though it is 2 sizes larger than you might think.

[Jinx]

I have 6 packs currently for testing, possibly more coming if I want them, however I would just like to prove the concept and get my head around setups. I’ve been asked to look into disposal for more but it’s very much on the drawing board and would require taking expanded cells and possibly burnt items, testing and sorting. I do have a few contacts from previously disposing of SLA batteries and all the paperwork it entails but need to look into LiFePO4 requirements.


I’m thinking a small inverter as house load is generally sub 500W with only part-time appliances tipping over. I probably will upgrade but accept in the long run it will cost more.

I have a 10mm supply out to the outbuilding where my solar currently is also, distance is approx 20m. I would like it in another building but that would require an upgrade.

[MrPablo]

I would echo getting a bigger inverter than you initially think.
I got a 3.6kW hybrid inverter which is fine for 95% of the time, but future plans involve a heat pump and all of a sudden I need to consider an upgrade.

[Mr Gus]

+1 Mr Pablo

[Jinx]

Seems pretty consistent advice, can you guys recommend a budget friendly one. I can always swap out my current Growatt for a hybrid if it’s really the best route?

[MrPablo]

If you're not fussed about getting a brand new unit, I could sell you my Solis RHI 3.6kW hybrid inverter.
I'm planning an upgrade anyway, so two birds, one stone.