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Maintenance charging solar battery bank: 8 lead-acid 6v deep

maine_tyler | Posted in General Questions on

Hoping the solar and energy storage folks here will have some advice.

I’m responsible for a seasonal woods cabin that runs off a small solar / battery bank set-up. It shuts down for the winter: the panels are small and not kept free of snow, and the storage location of the batteries is unheated (this is zone 6a- Maine).

So, the move is to pull the batteries into a heated space (basement) and maintenance charge them for the winter. 

The question is: Is it best to simply charge as a bank with one charger, identical to how they are arranged for solar? 
They are arranged as 2 parallel strings of 4 batteries in series—8 total.
Rolls batteries: 6V, flooded lead-acid, 230AH each. 

My assumption is to match voltage (so in this case 24V nominal) and use the existing wiring (even length legs for equalized resistance, charger hooked to opposite sides of strings).
I’m looking at a 3 stage, low amperage charger like this:

I mainly want to be sure that charging as a bank of 2-parallel-strings of 4-batteries-in-series is the right way to go. This is how they are set-up in the solar system. Is there any reason to break it up and charge them all in parallel (with a 6v charger) or invest in a multi-bank charger, or use multiple chargers, etc?

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  1. Jon_R | | #1

    Putting them all in parallel will assure that you get perfect voltage balancing - IMO an improvement over the usual small differences. With no load and low charging amps, don't worry about any difference in resistance between the parallel paths.

    I'm assume this is on-grid and there isn't any issue with the charger draining the batteries when there is no PV input.

    Maintenance charging algorithms vary - I'd look at this carefully. IMO, CTEK does a good job. I wouldn't use a fixed float voltage.

  2. Expert Member
    BILL WICHERS | | #2

    I don’t work with off grid stuff much, but I do work with VERY large battery systems at work. The telecom world runs on 48v battery stings made up of multiple series strings of 24 x 2v cells in parallel to arrive at whatever plant amp-hour capacity is needed, typically in the thousands of Ah range.

    I would leave the batteries in the string as they are and float charge them as two 24v strings in parallel. There isn’t really anything to be gained rearranging the strings for maintenance charging, and you potentially risk damaging the battery terminals every time you change a connection.

    That charger you linked to looks like it has a too-high float voltage for your batteries too, it’s probably intended for gel cell batteries. Flooded batteries need slightly lower charging voltages, you should be using around 26.4 volts with your batteries. Note that the optimal charging voltages are temperature dependent, so if you’ll be keeping your batteries significantly colder (or warmer) than the 78*F standard design temperature then you’ll need to adjust your float voltage for best battery life. The rolls surrette people can probably provide any info you need about this.


  3. maine_tyler | | #3

    Thanks Jon and Bill.

    Can these types of chargers typically be adjusted for optimal charge voltage output, or is the documented rating what you get?

    It will be on-grid charging.

    If there's no slam dunk reason to rearrange the bank, I'll probably just leave them as they were for simplicity and to utilize existing connections.

    I can't say I know how to effectively shop for the best charger, as there is a lot out there with similar looking specs. I've certainly no capacity to analyze their algorithms.

    Looking at information from Rolls, these batteries desire a float voltage of 2.25V per cell @ 77°F, so a sum total for the string of 27V. It'll likely be charged at slightly cooler ambient temps (probably 64-70°). Also not sure if any losses should be accounted for, but perhaps not at such low amperage.
    The originally linked charger does mention gel, AGM, but also wet.
    Lookign at this other charger:
    It states for 'lead acid' and 'excellent for use on deep cycle.' It has a float charge voltage of 26.6V (closer to what you recommend Bill). Maybe that's a better fit.

  4. tommay | | #4

    I would be careful leaving these batteries unattended hooked up to grid power for charging. It would be best to use your solar panels with a diversion load controller instead. If you can move your panels to a location where they won't get covered in snow for the winter, like under an eave or make a roof of sorts over them if you have them ground mounted. the winter sun angle should allow you to do this while still getting some sun exposure. Having a diversion load controller will make sure your batteries do not overcharge and possible create a dangerous situation. You could use a small heater as a diversion load to keep your batteries warm during the winter.

  5. maine_tyler | | #5

    The heater is an interesting idea.

    That aside for a moment: I'm not sure I really understand your concern with a 3-stage micro-processor controlled charger plugged into a ≈120v wall socket. I'm far from an expert on this, but aren't these chargers designed to sense the demand and adjust accordingly— I.e proceed through three varying stages of charge: bulk, absorption, then float—based on communication with the battery? The key being that charge isn't blindly pumped at full rates when the batteries reach full charge.
    This appears to be how they're advertised at least.

    I'll be keeping an eye on the electrolyte levels and have a hydrometer for checking specific gravity on occasion (probably just a couple times over the winter for verification of charge).

  6. Deleted | | #6


  7. Expert Member
    BILL WICHERS | | #7

    There is no need to worry about overcharging if you have the float charge voltage set correctly. The batteries can be left on a charger at float voltage indefinitely without any problems and this is the purpose of the float charge voltage. In the telecom world, we operate our DC plants (very large battery systems) on float charge for literally decades and power the equipment using the float voltage. In the telecom world, at least in North America (Europe uses a 60v system), we run about 52-54 volts (depending on battery chemistry) for our “48v” systems. They are positive ground systems too, so the red wire is the NEGATIVE lead. There’s the trivia question for the day :-)

    If you’re only float charging a relatively small battery string at s few amps, wire losses won’t matter much but you DO want to size the wire for minimum volt drop. It would be easier to just put the charger near the batteries though I would think.

    I can’t really recommend a small float charger. The systems I work with use devices way bigger than you need. If you are curious, I like the Lorain products (now part of Emerson electric), especially the flotrol designs. There Lorain flotrol rectifiers out there that are over 50 years old and still in 24x7 service. I think the smallest one they make is maybe 8 or 12 amps though. If you want to try one of those, they show up on eBay. REMEMBER they are going to expect a POSITIVE GROUND system which is the opposite of what most people will be using in their home systems.


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