Used AGM Batteries For Solar Power Energy Storage

As we extend the power system at Rally Creek, there are two primary components that will be expanded over the coming year. Power generation via new solar panels and power storage. We currently use Absorbent Glass Matt (AGM) batteries from companies like Renogy and Weize to store power. As we move forward we will be researching the viability of the “reuse” mantra in reduce-reuse-recycle. Part of that will be exploring used AGM batter for solar power energy storage.

While there are places to get used AGM batteries, they are not all viable for use as a storage solution. While some batteries may not serve their initial purpose, such as those in a vehicle that requires a high cold cranking amperage to push through that dummy-thick oil on a cold winter day, they will suffice for storing solar power. However some batteries are beyond usability as their cathodes, anodes, and plates break down or get coated with salts or other power transfer inhibitors over time.

We need to know if a battery is “good”, “bad”, or in between. Knowing little about the subject, let’s delve into some research and take notes here. Thus begins another “Rally Creek Notebook”, this time on AGM battery technology.

Our Battery Notebook

AGM vs. Gel Batteries

While shopping batteries it became apparent that some sites incorrectly reference AGM as Gel batteries and vice-versa. When building a battery storage array you want to keep the battery types the same. This ensures the charge controller can properly read and manage the battery charge properly. A charge controller needs to be set for the type of battery as AGM, Gel, LiFePO, and sealed lead-acid batteries all operate with different float and charge voltages.

While AGM, Gel, and sealed lead-acid batteries are all of the “lead-acid” family, there are some key differences. These differences are mainly related to construction and electrolytes used. AGM and Gel are “maintenance free” with no liquids to deal with like old-school lead-acid batteries (the kinds with caps you used to have to “fill up” with water to keep the electrolytes “topped up”. We are going to focus on AGM vs. Gel here due to the confusion on some sites.

Electrolyte Storage – AGM uses glass matts to act like a sponge that holds sulfuric acid and water. Gel us silica-based gel (think the beads in the small packs of desiccant) . The AGM battery thus tends to be “less sloshy” (that is a technical description) that Gel batteries.

Performance – AGM tends to perform better in rapid charge/discharge and have a high power density making the suitable for sudden power demands like a motor kicking on in a minisplit HVAC system. Gel are better for deep cycle use where they can be deeply discharged before recharging which can be the case with solar when power is consumed at night with no sun shining.

Temperature Sensitivity – AGM are less susceptible to temperature swings. Gel can degrade more quickly extreme heat or cold.

Which battery is right depends on your situation. If you think you can keep the battery “topped off” or near/above 50% state of charge most of the time, AGM may be a good choice. If not, Gel is probably a better option but only if the batteries can stay in a “comfort range” of not far below freezing and not much above a “hot summer day” (direct sunlight is probably not a good idea).

AGM vs. Gel Electrical Properties

This is an important topic for checking to see if used AGM batteries are viable for solar power energy storage. It also shows why you should not mix-and-match as the settings on a charge controller may not align with what the battery requires to stay charged.

Property	AGM	Gel
Boost (the big “push” to overcome built up resistance)	14.4 V	14.2 V
Float (the “trickle charge” to keep it topped off)	13.8 V	13.8 V
Low Reconnect (point at which the controller reconnects after low voltage state)	12.6 V	12.6 V
Under (critically low voltage warning)	12.0 V	12.0 V
Discharge Limit	10.5 V	10.5 V
Equilization	2 H / 30 D	N/A

Testing If AGM Batteries Are Bad

There is a great resource over at Renogy about testing AGM and Gel batteries.

The initial test is to check voltage. Keep in mind AGM batteries have slightly different voltage readings depending on the ambient temperature. As a side note, this is why many high end charge controllers come with a ambient battery temperature sensor. When the sensor is in place the controller can provide more nuanced voltage controls for the boost, float, and equalization stages of the battery charging process that can extend the life of the batteries.

Open Voltage Test

This is the basic test done with a multimeter. Set the meter to the lowest DC voltage setting above 12V, typically 20V. With no load on the battery (not connected, charging, trying to power devices, etc.) – touch the negative (black) lead to the negative battery post and the positive lead to the positive post. It should read close to 12.8 – 12.9 volts (depending on the outside temperature).

If below 12.8 volts it may need to be charged. Try charging the battery fully. There are AC chargers you can plug into if you have a grid-based source, or you can try the charge controller connection to the single battery if the voltage is above 10.5 V.

Other Tests

You can also perform load tests, resistance tests, and other checks however this is probably best done by bringing the battery to a local battery testing retailer like those at a car parts retailer or battery store.

In Summary

In general you may find used AGM batteries for solar power energy storage , especially if they are used car batteries that simply cannot perform under heavy load like cold-starting a vehicle. These batteries can have enough life to reliably stay over 10.5 volts when depleted and charge back up to near 13 volts and hold charge for 24+ hours. However, they must be tested.

Start with a simple multimeter open voltage test and if that does not provide clear “good to go” (12.8v+ fully charged voltage) bring it to a “battery shop” for testing. If the multimeter shows “good” but you are still not sure, you can have a load test performed at those shops, but keep in mind a typical tiny home or RV power draw (load) tends to be way less than the cold cranking amps most car batteries are tested for at a car shop.