I've got a small AGM battery (80 ah) for 12v DC house loads. I'd like to have this battery and the house panel segregated from the 48v propulsion bank but retain the option of charging the 12v battery from the 48v bank if necessary. I'll be using a 48v inverter from the propulsion bank for house AC loads. Given that AGM batteries like to be charged in stages, what is the best way to charge this 12v battery from the 48v bank on demand? I have a 3 stage smart charger for shore power charging of the 12v battery and it seems like it would be terribly inefficient to run that charger through the 48v intverter. Additionally I'll be adding a 12v solar panel strictly for the house battery to add additional redundancy. Can I simply use a 48v to 12v converter and attach that to a breaker protected circuit on the 12v panel which I can switch on as needed going directly to the battery? How can I ensure the 12v AGM is charged in the correct manner in such a case without a smart charger? Thanks for any insight you can offer.
s/v Miss Teak
Unfortunately there is no easy way to charge a 12 volt AGM battery without a "smart" charger. You need at least a three stage charger. You could purchase a DC-DC converter but you would need to carefully monitor the charging cycle by watching the volt meter and and the amp meter. It is very easy to destroy a battery with improper charging.
As you probably have already found, there is no readily available charger that will charge a 12 volt battery from a 48 volt battery. I had the same problem with trying to charge a 12 volt "house" battery from my 36 volt battery bank. I solved that problem by eliminating the 12 volt battery and just running my 12 volt system from the 36 volt by way of a DC-DC converter. I found a Chinese company that makes a 15 amp DC-DC converter for about $25. I think they also may a 48-12 volt converter. If I needed more amps I just put 3 of these in parallell to get 45 amps.
The other option I might suggest (though I have not tried it) is to look into getting a "solar" converter that charges 12 volt batteries from solar panels. These are smart chargers that can take a variety of input voltages - usually 36 -60 and keep a 12 volt AGM battery properly charged. You could treat your 48 volt battery bank as the solar panel. You would however need to find one that controls the input current so you don't burn out the converter. Most solar panels have a limited current and the battery chargers may not limit the input current.
Hope this helps.
There is a power loss doing conversion, plus the cost of the converter.
A no loss system & less expensive might be to eliminate the separate 12v cabin battery and simply wire into the motor bank directly with a multi-switch:
Assume you have a 48v electric propulsion made up of 8 6v AGM's wired in series as
1oo2oo3oo4oo5oo6oo7oo8oo9 where oo indicates a 6v AGM and #'s are the cables
Now add a multi-switch with positions: Off A B C D
Position A connect points 1&3
Position B connect points 3&5
Position C connect points 5&7
Position D connect points 7&9
You physically select which pair of 6v source you want.
Hmm, in that case I think I'd rather just keep the house load segregated. I want to make sure during an extended stay offshore that I have plenty of power in the propulsion bank if I need it and not have to worry about waiting to recharge due to use of nav lights, radar, etc. from the previous evening's passage. This is worst case scenario since we'll be carrying a small generator as well for the propulsion bank. But, generators break, petrol runs out or gets swept overboard, etc. As it is, we'll be pretty miserly with using the inverter if we use it at all unless we're at anchor and soaking up sunshine and/or wind. I'll use a small 12v solar panel (say, 60-100w) to top off the house battery and if I absolutely need to I'll tap into the 48v bank via a 48->12v converter and use the house battery's system monitor to charge manually. I'm thinking since we're mostly LED the only real power hogs will be instruments during passage and our electric toilet and electroscan which are high amps for short periods of time used mostly during the day. I'll probably keep my 12v smart charger wired into a circuit branch in the AC panel with the 48v charger on a separate branch. This way I can choose which bank to charge with the generator. Anybody else have a setup like this? How's it working out for you in practice?
I appreciate the responses and helping me think this through.
hi rolland if you look at my photos you will see two battery chargers one is 12 volt 50 amp for my house bank and the other is a 25 amp 48 volt for my engine. I run my honda 2000 when I go on trips if I find my self motoring all day do to no wind then I run the honda to put back some juice in the 48 volt bank,if i motor for appx 10 hrs while running my honda my 300 amp engine bank gose down to about 50% and its time to recharge the engine bank,that will take some time to get it back to 80% so far i have not had to take the 48vlt bank down that far. my house bank is 205 amp golf cart batts one set. if i run the 12 volt charger two times a day it will keep my bank at around 80% and that keeps up with the fridge,fan,and so on. so far on a week long trip this set up has done very well for me hope this helps bill.