By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
The sales guy at the local RV shop tells me I can just drop in a lithium battery to replace my pair of old Trojan lead-acid batteries, but I’m suspicious. Is there anything else I should check to make sure this works? Can I replace two 6-volt batteries with a single 12-volt battery, and will it give me more power, as this guy suggests? —Derick
Well, you’re half right. You can replace a pair of 6-volt batteries that were wired in series to make 12 volts with a single 12-volt battery, no matter what its chemistry (flooded cell, AGM or lithium). The power question is just a bit more complicated since you can discharge each of these battery chemistries to different levels without damage. But you’re also missing one important item on your checklist before you pull the trigger on this purchase and write the check – that’s battery charging profile.
That’s because each battery chemistry has a slightly different voltage and current rates it likes to be charged at. Go with too much current for too long and you’ll boil the sulfuric acid and water out of a flooded-cell battery and ruin it. Not enough voltage fed to a lithium battery and you’ll never be able to get it fully charged. And the latter is the potential problem with simply swapping in a lithium battery in place of a pair of flooded-cell lead-acid batteries.
It all comes down to what brand and model converter/charger you already have in your RV. Many of the modern ones from Progressive Dynamics and others have a switch setting on the controller board marked something like LI for lithium. If your converter has that switch, then you should be in business.
However, if it doesn’t and its technical manual doesn’t mention a lithium battery setting, then you won’t be happy since it probably won’t charge your new (and expensive) lithium battery to much more than 75% capacity or so. And since you’re asking the power question, I assume you want more usable power, not just expensive bragging rights.
Finally, on that power question, note that modern lithium batteries can be discharged down to 0% of capacity (flat-lined) without damage, so 100-amp-hrs of lithium battery is good for around 1,2000 watt-hrs of power. That’s because 100 amps x 12 volts x 1 (full discharge) = 1,200 watt-hrs. However, a standard flooded-cell battery should only be discharged down to the 50% level or you’ll kill it rather quickly. If you have a 12-volt, 100-amp-hr battery it can provide only half that amount of power. That’s because 12 volts x 100 amp-hrs x 0.50 (percentage of discharge) is only 600 watt-hrs.
In your case a pair of 6-volt batteries with a 225-amp-hr rating will provide around 1,350 watt-hrs of power (because 12 volts x 225 amp-hrs x 0.50 = 1,350 watt-hrs). So you would be losing a little bit of total watt-hr capacity with a single lithium battery. However, if your converter/charger has a lithium setting and you could afford to put in two lithium batteries, you would now have a whopping 2,400 watt-hrs of usable power.
So you need to do a little more homework to determine if you also need to change out your converter/charger, and if you can get by with a single lithium battery or need to put in two of them to get the power you’re wanting.
Hope this helps your decision process and doesn’t damage your checkbook too badly….
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 Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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