last stand of the GC2

Spoiler: I just ordered a 100Ah LFP from Rebel Batteries.


3 years ago I bought the cheapest GC2 available (Duracell by E. Penn), 220Ah for $200 at Batteries Plus. I think they are hitting the wall.

I wasn’t paying attention to current acceptance in Absorption early on and ended up under-charging them the first two years. My controller maxxes out at 3 hours and the bank takes more like 4-5 hours of absorption to drop to C/125 (1.75A).

signs of distress

In the last week I’ve started to see hints of degradation:

  • slightly lower voltage after sundown
  • significantly higher acceptance current - doesn’t seem to want to drop below 3.0A.

Normally I would associate this with insufficient solar exposure but I am in Santa Fe and have been enjoying sunny skies. Right now after 7 hours at 14.7v the bank is still struggling to get below 3A acceptance. I imagine they have been fading and just caught my attention now. In restrospect they did drink a bit more last month.

let’s check it out

I just watered the bank a month ago but I’m going to take a peek anyhow. If that 3.0A really is the new bottom then the bank is probably using more water.

Temp-correctred hydro reading is 1.260 even though it’s sitting at what most people would call “fully charged” (Vabs 14.7v for 4 hours at the time of sampling). Cell readings were consistent between them, so I suspect systemic sulfation more than a damaged cell.

The cells were surprisingly thirsty, taking as much after 4 weeks as they usually do in 8-10 weeks. Uggh. This isn’t looking good.

duty cycles

Documentation is tough for these batteries. Closest to a cycle rating I can find is 1,025 @ 30% DoD. Last night was cycle 1,099 at discharges between 20-50% so I appear to have beat the specs, even with 2 years of suboptimal charging.

I suspect with correct charging from the beginning the bank would have gone 1,500+ cycles.

what to replace them with

The main issue is whether to replace them with the same or just do LFP. 100Ah of LFP would be fine for my needs. Even 50Ah probably would, but that would limit me to 25A charge/discharge for longevity.

The main benefit of LFP in my use would be the able to use campsites that have imperfect solar exposure – no pressure to get to 100% SoC every day. Just need a few hours of sun to make the nut. It would also allow me to run loads when I want rather than when there is excess solar; borrow against the lithium, so to speak, as long as they stay above 20% DoD. .

Another problem is timing. I will be moving around the country for the next 12 months and would rather deal with the issue now if it’s mandatory. I am near my home base (El Paso) and will be able to receive packages there (including LFP).

Rebel Batteries

The normal 100Ah batt is $559 as of this writing. The closest analog is the SOK for $570.

They got a test batch in with a non-waterproof switch and 5P4S instead of 1P4S, using the same excellent BMS with cold-charging shutoff. They are going for $440 + free shipping. No, I don’t have an extra $440 laying around, but neither do I have an extra $200. Investment in the future, I guess.

The warranty is for 5 years, although I don’t have much truck with warranties. Companies can go away.

The battery specs say 4,000 cycles at 80% DoD which matches my understanding of LFP longevity at different DoD. In order for the batt to match the value of the old GC2 it will need to make it to at least 2,462 cycles, about 6.75 years.

what I like about Rebel

  • Will’s teardown looked good with some teething pains. Good BMS (most likely point of failure)
  • the owner responded appropriately and quickly to the issues
  • the owner has continued to respond to other suggestions from users (like changing to female battery terminals - the owner of RB is ashelby in that thread.
  • I made a pre-sale inquiry 5 months ago and received a timely and useful answer.
  • their YouTube channel is useful and BS-free.

BTW, I made a ticket to provide information on my order and it was answered within an hour on a Saturday afternoon.

what I don’t like about Rebel

lack of specific charging info

I’d like to see better documentation about charging. On this page They specifically recommend this shore power charger. According to the specs (pdf) it charges at 20A until reaching 14.4v, at which point it drops back to 14.3v-14.4v @ ~2A. “Floating” LFP at 14.4v makes me nervous.

On this page they recommend a 20A-40A DC-DC charger and link to a video that demos the Renogy 20A version. According to the manual (pdf) the lithium profiles for LFP charge anyhere from 14.0v to 14.6v.

I suppose we could deduce from that Rebel is ok with 14.0v-14.6v quasi-Absorption and 20A-40A charging rates. They seem to have a “throw what you want at it and the BMS will sort it out” approach.

The BMS shuts off for high voltage at 3.65vpc (14.6v) and considers the bank 100% SoC at 3.4vpc (13.6v).

RB is a lithium company and it’s right to expect them to know about that chemistriy. It’s ok if they don’t know about lead deep cycles as long as they don’t try to hold forth on it.

Consider these unforced errors.

The problem with lead-acid chargers is that they run a “charge profile” and vary amperage based on the voltage of the battery. (from this page

Not really. They hit (or attempt to hit) certain voltage setpoints and the battery bank determines how much current it will accept.

as the voltage increases the charger will begin to send fewer amps to the battery, this is called the “absorption” phase

Fundamental misunderstanding of Absorption. The controller holds Absorption voltage (Vabs) and the battery current acceptance tapers off as the bank approaches 100% SoC. The controller reduces current supply to match current demand. Otherwise Vbatt would rise.

Finally, once the charger gets near a predetermined voltage of about 13.1 volts, the charger will transition into a “float” stage where it just barely keeps any amperage on the battery to keep it “topped off”.

I produces the amount of current demanded at that time. The battery is in charge (so to speak!)

A lead-acid charger will typically charge a LiFePO4 battery 4-5 times slower. That is obviously a huge disadvantage of using a lead-acid charger on LiFePO4 batteries

No, it won’t.

Consider a lithium bank that needs 50A replacement amps to get back to ~14v. We have the 20A Lithium charger above and a 20A solar controller with enough grunt to get the job done.

  • Lithium on a 20A “lithium” charger = 2.5 hours (50Ah/20A)
  • Lithium on a 20A “lead” charger = 2.5 hours (50Ah/20A)

Wow not much difference there. Like none. The differnce is what happens after hitting tht 14v. The “lithium” charger listed would hold the bank at 14v (bad, IMO) and the lead charger would let it fall back to a resting voltage (good, IMO).

The next issue is that even at 13.1 volts, a LiFePO4 is not fully charged. A fully charged LiFePO4 sits around 13.6 volts. Between 13.1V and 13.6V is a pretty good bit of power and worth noting that you won’t get 100% of your available power if charging with lead-acid chargers

The lead charger charged to whatever voltage you wanted, say 14v. It was fully charged. Float at 13.1v is the fallback voltage the solar will hold the bank at after full charging. Resting away from 100% is actually good for lithium. If you want 13.6v then set Vfloat to 13.6v.

Which brings us to the one lithium-related error I’ve found so far:

You’re next question might be “Isn’t it better to charge slower?” No, not with LiFePO4, there are no negative effects of charging at high charge rates on LifePO4 batteries.

There are substantial benefits to charging/discharging at lower rates. C/2 (50A for 100Ah bank) is a common max rate if one wants to extend longevity.

charging lithium

I have started a separate page that describes my LFP charging regime.


Just before I publish this the bank has been at Vabs for 7.5hrs and still >3A acceptance. Definitely limping.


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