Questions about how to safely charge AGM batteries (disconnected from system)

1911 · May 11

I’m new to solar an electricity. I have been buying components for a system but have not hooked it up yet…still learning and researching.

I bought a pair of SunXtender AGM batteries, 12v, 153ah each. I also have a pair of Iota DLS-30 (30amp, 12v) battery chargers. ( i have the AGM plugin)

It will be a while before I put this system together. I feel like I need to hook the chargers to the batteries to keep them charged and healthy.

Questions:

1. is it safe to charge these in the house…or would you put them in a hot garage?

2. Can I hook the charger up directly to the batteries? Positive to positive, negative to negative? Any kind of ground required? (The reason I ask this is because I ran across an automotive forum and they made a big deal of hooking up negative to the frame so it didn’t spark and potentially ignite hydrogen gas)

3. i also have a cotek 24v battery charger, can I hook up two 12v batteries in series for a 24v battery bank and use the 24v charger to charge them both at the same time?

4. Do AGM batteries off gas much?

5. would you feel safe to use charge these in your home unattended?

6. Would you leave them hooked up all the time, or just hook them up for a day every few months?

thank you!

Wheelman55 · May 11

One of the experts will likely chime in soon.

I think (non expert) that you’ll want to charge each battery to 100% SOC before hooking them up in series.

Positive charger to positive battery, negative charger to negative battery is correct.

I would charge them every few months.

The experts will comment soon.

Best of luck with your future system.

BB. · May 12

As always, read the manual.

https://www.solar-electric.com/lib/wind-sun/techmanual.pdf

To add to what WM55 said:

Questions:

1. is it safe to charge these in the house…or would you put them in a hot garage?

AGM batteries are relatively safe to charge in your home... HOWEVER, if the charger "goes bad" (over voltage) or when the batteries age/cycle out--They do eventually gas/vent (pressure relief valve for each cell)... Hydrogen+Oxygen gas, and possibly some electrolyte (sulfuric acid + water) mist--Again, this is when the batteries are getting/approaching failure. Normally, they will not vent when charged according to the manual. Once they begin to vent/lose "water", that is down hill for them (AGMs are not supposed to be refilled with distilled water).

Heat is bad for most batteries (and heat/thermal cycling is pretty hard on almost anything from batteries, food, electronics, etc.).

The "normal" assumption is storage at 75F for "spec'd aging life" and most electrical specifications.

There is a rule of thumb in engineering for every 10C/18F over "room temperature", the "thing" is expected to age 2x faster (or 1/2 the life). Conversly, if you store at 10C/18F under "room temperature", the "thing" will age 1/2 as fast or last 2x longer. Again, this is the math and a recommendation--Not written in stone.

AGM batteries are more efficient at charging, so they do "better" in a warm space--They do not heat as quickly as standard flooded cell lead acid batteries.

2. Can I hook the charger up directly to the batteries? Positive to positive, negative to negative? Any kind of ground required? (The reason I ask this is because I ran across an automotive forum and they made a big deal of hooking up negative to the frame so it didn’t spark and potentially ignite hydrogen gas).

Grounding is a "complex subject". The batteries will work fine if grounded or not. Grounding has several uses... One is if you ground the electrical system to "frame/plumbing" ground... This prevents "metal things" (doors, sinks, etc.) from becoming energized (+12 VDC, 120 VAC, etc.) and electrocuting somebody or other sparks/short circuit fires.

Ground bonding also allows us to use breakers/fuses on the "hot leads" and not have any breakers/fuses on the return/ground leads. If you have a "floating power system", generally you are supposed to have fuses/breakers on both the hot and return leads to protect against "all possible" short circuits.

Grounding, for example, does not "prevent" sparks. Anytime you are working with Lead Acid Batteries, there is always the chance of hydrogen in the area (from charging/EQ Charging/gases in cells after charging). Wearing goggles, safety gear, protecting wrenches/tools with tape (to prevent a wrench on +12 from shorting to -12/frame ground etc.). Same thing with jewlery--Remove rings and other metal before working on electrical power systems. 12 volts is relatively safe (will not normally electrocute you)--But Large AGM and FLA (and other) batteries can output 100-1,000's of Amperes. Very easy to "spot weld" a wrench to the frame, turn wiring red hot (of course, fuses/breakers are there to protect wiring).

3. i also have a cotek 24v battery charger, can I hook up two 12v batteries in series for a 24v battery bank and use the 24v charger to charge them both at the same time?

As Wheelman55 says, batteries should have "equal" state of charge before placing in series so that they share equal energy storage/supply. AGM batteries do not really EQ charge well (balance low and high cells in a string with "selective over charging". FLA batteries cells can be "over charged" (within reason) and bring up the "low cells" in a series string.

4. Do AGM batteries off gas much?

AGMs in "good condition and operated withing specs" do not (generally) out gas until end of battery life (when charging).

Will you 'catch' the batteries "before end of life" failings? Or would it be better to install them in a "safe space" always in normal operation... I.e., we have had people ask about installing AGM batteries under beds or in RV sleeping spaces, etc. Having sheet rock around battery box (as an example) to limit flame spread is not bad... Also if under a bed, make sure that the box springs cannot fail and short out the exposed battery connections (larger batteries can easily turn the box springs red hot during a short circuit).

5. would you feel safe to use charge these in your home unattended?

Charging (not old) batteries in a garage with a known good battery charger--Everyone pretty much does this. Placing batteries on a non-flammable surface and away from "nice electronics" (to avoid possible out gassing fumes) does not hurt either.

6. Would you leave them hooked up all the time, or just hook them up for a day every few months?

AGM batteries are typically rated to self discharge to approximately 75% State of charge between charges at 6 months at 75F. That is the typical assumption when AGMs need to be recharged (I.e., 75% SoC and below, they begin to sulfate more quickly).

And taking the 10C/18F rule of thumb into account... If the batteries are stored at 92F, they will self discharge 2x faster--Or need recharging in ~3 months.

In general, batteries begin to "age" the moment the chemicals are mixed together with the plates. Buy batteries when you need them so they will last the longest in your applications/needs.

-Bill

1911 · May 12

Thank you both for the great answers!

Marc Kurth · May 12

How old are the batteries? If not new, what was their previous application? Date code?

What is the charging voltage of the IOTA? Last I checked, they do not temperature compensate.

After fully charging, disconnect and let them stand for a few hours.Then take an accurate voltage reading to see where you are. This is important.

Marc

Questions about how to safely charge AGM batteries (disconnected from system)

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