Will my lead acid battery freeze this winter?

Not if you’re careful to disconnect parasitic loads…

Welcome to my J.A.M. (Just Ask Mike) Session, a column where I answer your basic electrical questions. If you’re a newbie who’s never plugged into shore power (or ask – what’s shore power?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you.

Send your questions to me at mike (at) noshockzone.org with the subject line – JAM. Today I discuss using and/or storing lead acid RV batteries in the winter.

Dear Mike,

I have a truck camper that is used for camping most of the winter. It has a standard flooded cell battery. The outside battery compartment, while allowing for off-gassing of the battery, also allows the zero degree temperature to affect battery performance and potential freezing. I understand that it is potentially unsafe to place the battery inside the camper … but how else can I keep it from suffering/freezing? Or is there a safe way to store the battery inside the camper while in use? —Reed

Dear Reed,

I found this information on the U.S. Battery website:

A FULLY CHARGED LEAD-ACID BATTERY HAS A FREEZING POINT AROUND -80 °F.  AT A 40% STATE OF CHARGE – THE ELECTROLYTE WILL FREEZE IF THE TEMPERATURE DROPS TO APPROXIMATELY -16 DEGREES F – WHILE A FULLY DISCHARGED BATTERY HAS A FREEZING POINT AROUND +20 °F.

Temperature of Storage  

The warmer the environment while a battery is in storage, the faster the rate of self-discharge. For example, a battery being stored at an average temperature of 80℉ will discharge at a rate of 4% per week. Whereas a lead acid battery being stored at 65℉ will only discharge at a rate of approximately 3% per month.

What does this mean for your RV battery?

So, if you have a standard 3-stage charger/converter on the battery, it will keep the battery fully charged as long as your RV is plugged into shore power. And, as noted above, a fully charged lead-acid battery is safe down to -80 degrees F. Let’s hope you’re camping in warmer weather than 80 degrees below zero.

Disconnect parasitic loads

And if you need to unplug from shore power for a month, as long as you use the battery disconnect switch and make sure that your Smoke, CO and LP gas detectors aren’t pulling any current while in storage, you should still have plenty of charge left to prevent the battery from freezing. That’s because a FLA (Flooded Lead Acid) battery will generally self-discharge at a rate of around 5% to 15% per month (depending on storage temperature). So you should still have at least 85% SoC (State of Charge) after a month of sitting disconnected from everything.

But what if I’m shutting down for the season?

However, if you need to leave your RV unplugged for several months or more, then I think that a Solar Battery Tender is a great option. Be sure to get one with a built-in battery maintenance function that will keep your battery a safe charge level, not just trickle charge your battery. You can find one on Amazon HERE.

Check your battery water

And be sure you check the battery water level every month if you’re plugged into shore power and using the RV, and certainly before you put your RV to bed for the winter. You’ll want to top off the battery water level first (but don’t overfill it) and then do any final charging to make sure the water and sulfuric acid have a chance to mix.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there…. Mike