How can i add a second battery?

ok maby i spoke too soon on rv"s i have no hard proof but  i will do my research

Answer this for me.. how much power is coming _out_ of a battery when the system voltage is 13.6vdc?

ok if your asking me how much power you can get at 13.6vdc  that depends on the load. with out you giveing me your amperes or  the resistance of a load i can not tell you how many watts you batt. is putting out

When system voltage is 13.6v, the battery is being charged and nothing is coming out of it. I can take a 12v 7ah battery and plug it into the power port on the dashboard (of my work truck) and charge the battery. The fact that it's in parallel with a 12v 105ah battery isn't important.. because they are both being charged.

Discharging is a different matter entirely.

As I mentioned earlier in the thread, how you hook up two (or more) batteries depends on what you are attempting to do. Ford uses two 12v batteries in parallel in their diesel pickups; they need the cranking amps. These do not use an isolator, and they need to be matched batteries. Military equipment is often 24vdc; they use two 12v batteries in series, no isolator. These also need to be a matched set of batteries. Early Porsche 911/912 used dual batteries.. again, matched set.

If you want a second battery to run your stereo/ham radio/inverter/whatever with the engine off (and you don't want to kill the starting battery) then use an isolator and a deep cycle battery. Discharging two paralleled batteries of different capacities just isn't a good idea. Deep discharging a non-deep cycle battery (like a starting battery) isn't a good idea either.

Jim

Did the RV tech mention anything about the different sized batteries being charged?..  Solid state isolaters are nice (and expensive) but did he really say that they switch the alternator output from battery to battery? Isolators divide the charging output.. not switch it from battery to battery.

There are some isolators/combiners that will delay charging the coach battery bank if they detect that the starting battery voltage is low.. but once the starting battery is charged the isolator/combiner goes back to charging both at once.

Jim

i cut this from http://www.hellroaring.com/rv.htm

For motorhome types with high output alternators and large house loads while operating, consider the RV starting battery isolation technique as a simple solution to multiple use batteries.  With this setup, a high output alternator can supply maximum charge current to large house battery banks.  All batteries receive proper alternator voltage from an internally sensed alternator due to the very low ON resistance of the BIC-75150A (less than 0.008 ohms).  Since there is no diode drop, the starting battery stays fully charged.  Electrical loads on either battery will not draw from the other.  If you choose this method, we recommend that you contact our technical support staff for assistance.

For most applications, (i.e. slide-in camper, trailer, RV's with required continuous loads under 50 amps on the auxiliary side during engine operation, etc...) consider the basic deep cycle application.   Your primary objective is to prevent your  starting battery from draining while you consume power from your house (deep cycle) battery.   (Don't be fooled by low cost battery disconnect devices as a solution.  If you try to use these, your starting battery will get deep cycled and will result in early failure.)    You also want your house battery to obtain full charge as quickly as practical while operating your vehicle.   With the BIC-75150A installed as shown below, you will obtain those two objectives!   The BIC-75150A has very little resistance (less than 0.008 ohms) when ON during charging and it automatically isolates the two batteries when charge voltage stops, even if your alternator fails.   If desired, you will have the ability to combine the batteries manually for a starting boost (the main battery must still supply substantial cranking current).   It has no mechanical moving parts to degrade or wear out!  For those on a budget and who want the simplest installation, the BIC-75150A basic Auxiliary setup makes an excellent choice for full charging without the voltage drop or bulky physical size associated with diode isolators!  Works great for alternators sensed at the alternator output (internal sense.) 

I had looked at that company.. it's not a diode based device, and it's not a solenoid either; I don't know what technology they're using.  I did read their technical data.. it looks like the device combines batteries when charging voltage hits 13.4v, and disconnects them at 13.2v

The voltage drop across the unit is extremely low.. I can't recall enough electronics theory offhand to remember if a power transistor has a voltage drop associated with it; if they weren't saying that there's not moving parts I'd say it's a relay inside. There are some current limitations on it.. the model you listed has a continuous limit of 50 or 75 amps I believe..

"Electrical loads on either battery will not draw from the other"..

If this device opens the circuit at 13.2v, of course they won't.. I mentioned that earlier. But they do mention that you can add a switch that combines all batteries for an added boost for charging.. which means the unit _does_ hook all the batteries in parallel when operating.

Did you see anything about batteries needing to be identical in age, CCA and type?

Jim

you wrote. " I had looked at that company.. it's not a diode based device, and it's not a solenoid either; I don't know what technology they're using." as far as i know what technology thay are use is as it has allways been with solidstate relays (solidstate = no moving parts), my boss said he used to ues them in the eary 90's in show and comp cars.

"Did you see anything about batteries needing to be identical in age, CCA and type?"

no i did not. from what i understand their product allows you to hook up odd battries.

correction. you said " But they do mention that you can add a switch that combines all batteries for an added boost for charging.. which means the unit _does_ hook all the batteries in parallel when operating. "

thay said  "If desired, you will have the ability to combine the batteries manually for a starting boost (the main battery must still supply substantial cranking current."

But they claim there's no voltage drop with their device.. research a solid state relay and see what kind of voltage drop you see with those. 

I did mean 'added boost for starting'.. sorry about that. If you can combine the batteries manually, it means that the device will allow backfeed from one battery to another. I suspect that this device also charges both batteries at the same time, ie: they are connected in parallel.

Personally, I have some doubts about the hellroaring stuff.. They never mention the technology they use, and the specs they provide don't seem right to me. They have heat sinks; evidently they are generating heat, which is power loss.. the one device dissipates 36 watts at 120 amps.. do some Ohm's Law (W=V*A) and think about it.

Hellroaring also sells solid state relays.. but they don't list a voltage drop spec.. everyone else does, but not Hellroaring

http://relays.tycoelectronics.com/datasheets/DS11-1006_ds_7-1773440-9.pdf 

But the main point of this discussion was hooking unlike batteries together, was it not? At this point, I haven't seen anything that warns against hooking different batteries together if you are using an isolator of some sort.

Jim