Earth Notes: Off-grid Battery Bank Replacement (2021)

Updated 2021-05-15 18:21 GMT.
By Damon Hart-Davis.
10 year old battery bank is at about 10% of original capacity; aiming for a simpler and smaller and cheaper replacement.
20210515 battery bank 12V lead acid sealed 4 gel 99Ah
A decade ago I upgraded from a 40Ah battery to a ~400Ah battery bank, which is now worn out. But I don't think that I can really use that much capacity even with the huge increase in PV. So I'm going for a smaller and less complex battery this time around, but still lead-acid. Lead-acid works, is recyclable, and I don't yet trust or understand lithium BMSes!

2021-05-03: New Battery?

Yet again, I'm considering replacing my gel LA (lead-acid) set now over 10 years old.

I don't think I'll go for another ~400Ah because I don't think the charging energy is there in winter to justify it. With a minimum load of the RPi(s), much less than 100Wh/d capacity is needed to carry it, maybe only 50Wh/d. A completely sunless week suggests ~500Wh required usuable capacity, ie down to ~50% DoD. That could be ~100Ah at 12V.

(If I do have excess above the bare minimum then taking other loads off-grid such as ~300Wh/d of router is good. But 50Wh/d to move off-grid through the evening peak is also acceptable.)

2021: ageing 12V 4x 99Ah gel battery bank
Ageing 12V 4x 99Ah gel battery bank after 10 years' service.

One suggestion is a drop-in single Victron Energy GEL Deep Cycle Battery 12V 220Ah BAT412201104. (£429 incl VAT, shipping £22 incl VAT.)

Another more adventurous suggestion c/o "Scruff" is one (or two) Firefly 12V 116Ah G31. (£410 each incl VAT and shipping.)

If you're shallow discharging then one firefly with 116Ah has more energy over ownership than a Victron 220Ah (you can really "abuse" firefly, 80% DOD is perfectly acceptable).

In this case Scruff is suggesting setting the SS MPPT to flooded mode, but with equalisation turned off. Maybe the internal setpoints could be tweaked a little bit too. In particular the SS MPPT temperature compensation is -31mV/°C, vs the Oasis suggested -24mV/°C. (Both with a reference of 25°C.) Scruff created an MSView file to fix them. DIP switch 1 has to be on, and the jumper inserted.

Given my estimate of the current old set current usable capacity of ~300Wh, a single Firefly at ~700Wh usable would be an improvement.

There's some other minor issues like needing new M10 cables unless I drill out the old ones. And disposing of the old batteries (and my original 40Ah!)...

(Shipping is included in the Firefly price, so I could order a first, get the initial cabling right, then order a second soon after and get the cross-straps in place.)

Maybe the end of summer is the time to do it, given that there is less work for the batteries to do until autumn.

2021-05-14: Tanya and Victron Gel

I made a gentle enquiry with Tanya about the Victron Energy GEL Deep Cycle Battery 12V 220Ah BAT412201104.

The responses were polite and fast and informative!

Tanya said that there would be a fixing kit, nuts/bolts with the battery, and that my existing M8-lug cables should be fine. Lead time is currently 7-14 working days from order. But any date up to autumn would be fine.

The advantages of this less exciting option over the Firefly might be:

  • The "bank" would be just one battery with no cross-wiring.
  • The solar controller settings can remain unchanged on "gel".
  • Most or all of the tuned parameters in my powermgr code can probably remain unchanged.
  • No new cables to buy or route.

Given that I don't know how long we will remain in this house given the estate redevlopment, maybe this isn't the moment to try to maximise longevity.

The Firefly would however give me experience of novel AGM, so maybe I should seek out the opportunity to do that elsewhere if not here.


~756 words.