Weather the Storm with Backup Power – Part 2


by Sarah Latimer, Survival Blog:

(Continued from Part 1.)


At a high level, the stages of charging a battery include: bulk, absorption, and float. On batteries that have been discharged deeply, there is also an equalization charge required.

Bulk charging demands high current. Absorption charging requires less current but a slightly elevated voltage. Float is your trickle charge which has low current at about one volt above the stasis voltage of a charged battery.


As covered in a recent SurvivalBlog article, the charge levels of flooded batteries can be determined accurately using a hydrometer. Each cell should have an equal level of charge. If the levels differ, it is time to equalize them. The equalization charge requires a voltage about 10% higher than what is standardly used for absorption. Equalization time will vary slightly, but generally it is one to two hours. With experience, you will learn whether equalization needs to be performed monthly, or more frequently. In the case of infrequent discharge cycles, it might even be sufficient to equalize annually.

Flooded lead-acid batteries emit hydrogen gas while being charged. Manufacturers recommend that enclosed spaces containing such batteries be adequately vented to the outside and that you don’t cause sparks nearby.

I have experienced explosions while charging a neglected battery – don’t be careless. If you are also a compulsive experimenter at heart, you might be interested in the observation that I keep my heavily used wood-fired cookstove in the same 14-foot by 16-foot room as my flooded deep cycle batteries. However, I do not equalize batteries when there is a fire going. My batteries are in a well-vented box designed to help contain possible mishaps.

If you have safety concerns about gassing, you ought to read carefully about sealed lead acid batteries which are becoming increasingly common. The absorbed glass mat (AGM), has similar charge characteristics as the flooded lead-acid battery, which means that most chargers designed for flooded batteries will work adequately with AGMs. The more sophisticated chargers further have the ability to tweak the charge regime slightly to accommodate the subtle differences. Differences may be subtle, but accommodating them could extend the useful life of your storage devices.

A key advantage of the AGM is that as a sealed unit it can be placed on its side without compromising its effectiveness. AGMs also can be used without consideration to venting. They are typically sold as 12-volt storage devices and while they do tolerate moderately deep cycles, they are also reasonable at providing the instantaneous high current required by vehicle starting requirements. Many high-end modern cars are now using AGMs in place of the flooded variety. AGMs are significantly more expensive than a standard flooded deep-cycle battery.

There are other lead-based batteries around, like the gel cell – but mostly these have been replaced with the slightly more robust AGM. Be aware that gel cells can be destroyed by high-voltage charging. Follow the battery manufacturer’s instructions to be sure.

There are also those batteries sold as “Marine/RV deep cycle” batteries. These are not truly deep cycle and are not truly starting batteries – but might work adequately as either.

Some still might argue that the lithium chemistry batteries are more efficient, but is this like saying that the Lincoln is a better car than the one you are currently driving?

Most manufacturers rate the AGM and the deep cycle flooded batteries at better than 90% efficiency. If you take care of the batteries, they will continue to operate at better than 70% efficiency for more than a decade. If you abuse them, of course, they might last much less.


The most common abuses of a lead acid battery involve neglect.

All batteries self-discharge over time. If you allow the battery to sit for weeks never topping up the charge, the lead acid battery will sulphate and shorten its useful life. With vehicular batteries we rely on trickle chargers, on battery maintainers, for vehicles which see patterns of seasonal use. Some of these trickle chargers are small solar panels that sit on the dash. While these were once the rage, their solar panels were often too tiny and inadequate to be of much practical use.

A second abuse is allowing the electrolyte levels in flooded lead acid batteries to go below the level of the plates. Periodic maintenance of electrolyte levels is essential. If you can’t discipline yourself to this routine, then you might consider spending extra money on AGMs.

A third abuse is allowing the discharge cycle to go too deep. Unlike the computer controlled systems required for lithium, lead acid technology leaves it up to you to avoid excessive discharges. A shunt-based battery monitor is the most convenient way to keep track of battery state of charge and to guard against excessive discharge. Without a monitor, you will rely on regular hydrometer readings, intuition and battery voltage to estimate the state of charge.

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