Sizing battery banks and converter
I am planning to upgrade the 12V DC system of my RV. There are of course many practical considerations to be taken, but the focus of this post will be only on sizing of the batteries and the converter, while ensuring good charging of the batteries, the current draw of the converter and the theory behind it all.
This is NOT a discussion about different battery types, type and use of 12V applications, practical considerations of battery placement or battery care.
So, here we go: I would like to double my battery capacity, by doubling the number of batteries in my bank. The converter installed in my RV (and the alternator) SHOULD deliver enough current to charge this doubled battery bank. It will of course take longer to reach a full charge, and I am considering how much time I can accept to reach full charge.
My first question is: When is the converter too small? I've read that you get the longest battery life when the batteries are charged and discharged slowly. From that perspective, batteries in a big bank charged by a small converter should last longer. On the other hand, if the converter is too small for the battery bank it may not be able to equalize the batteries. I've seen recommendations that to ensure equalization, the converter output (subtracted for the RV's DC loads) should not be less than 5 percent of the batteries Ah rating, and ideally not less than 10 percent.
The next question is: When is the converter too big? I am considering upgrading my converter as well, to reduce the time needed to fully charge the batteries. I dry camp often, and prefer to run my noisy, polluting generator as little as possible. One reason why I am reluctant to up-size the converter is that I will increase the 120V maximum power requirement of my RV, and with the AC running, I don't have too much to spare.
But now let's say I decided to go for a really powerful converter. I have seen advice against doing this. But I cannot understand why? Think of what a converter can do and what it cannot. It CAN of course control the voltage that it is charging the batteries with, and as we all know, it changes this voltage during the different stages of charging. What it CANNOT do is control the current going into the batteries during charging. We all remember Ohm's law: The current is the voltage applied to the load (in this case the battery), divided by the resistance of the load. So the batteries should be charged the same way with a very powerful converter, as they would with a simply sufficient one. They will be charged with the same current.
Now please, keep this thread factual and on topic! Insights into how the batteries work during charging and discharging, at different SOC's, etc, are most welcome. So are insights into what the chargers do during the different stages of charging. Links to good write-ups explaining the theory are gold!
If you would like to share with the community your thoughts on solar, shore power, LP, generator types, battery types, ideas for how to use your available power, please do so, in another thread...
Please DON'T tell me about YOUR RV, the specifics of it and how you are using it.
Thanks!
This is NOT a discussion about different battery types, type and use of 12V applications, practical considerations of battery placement or battery care.
So, here we go: I would like to double my battery capacity, by doubling the number of batteries in my bank. The converter installed in my RV (and the alternator) SHOULD deliver enough current to charge this doubled battery bank. It will of course take longer to reach a full charge, and I am considering how much time I can accept to reach full charge.
My first question is: When is the converter too small? I've read that you get the longest battery life when the batteries are charged and discharged slowly. From that perspective, batteries in a big bank charged by a small converter should last longer. On the other hand, if the converter is too small for the battery bank it may not be able to equalize the batteries. I've seen recommendations that to ensure equalization, the converter output (subtracted for the RV's DC loads) should not be less than 5 percent of the batteries Ah rating, and ideally not less than 10 percent.
The next question is: When is the converter too big? I am considering upgrading my converter as well, to reduce the time needed to fully charge the batteries. I dry camp often, and prefer to run my noisy, polluting generator as little as possible. One reason why I am reluctant to up-size the converter is that I will increase the 120V maximum power requirement of my RV, and with the AC running, I don't have too much to spare.
But now let's say I decided to go for a really powerful converter. I have seen advice against doing this. But I cannot understand why? Think of what a converter can do and what it cannot. It CAN of course control the voltage that it is charging the batteries with, and as we all know, it changes this voltage during the different stages of charging. What it CANNOT do is control the current going into the batteries during charging. We all remember Ohm's law: The current is the voltage applied to the load (in this case the battery), divided by the resistance of the load. So the batteries should be charged the same way with a very powerful converter, as they would with a simply sufficient one. They will be charged with the same current.
Now please, keep this thread factual and on topic! Insights into how the batteries work during charging and discharging, at different SOC's, etc, are most welcome. So are insights into what the chargers do during the different stages of charging. Links to good write-ups explaining the theory are gold!
If you would like to share with the community your thoughts on solar, shore power, LP, generator types, battery types, ideas for how to use your available power, please do so, in another thread...
Please DON'T tell me about YOUR RV, the specifics of it and how you are using it.
Thanks!
