3 battery isolation
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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=123102
Printed Date: April 28, 2024 at 8:58 PM
Topic: 3 battery isolation
Posted By: hawks
Subject: 3 battery isolation
Date Posted: August 13, 2010 at 12:09 AM
I am trying to isolate 3 seperate batteries on one rig, my starting battery my AUX battery and my camper battery. Currently the starting and AUX battery are isolated with a solenoid but I want to isolate my camper battery from the rig batteries. Can/should I do this with the same solenoid or can/should I install one solenoid after the starting battery then continue the lead to the AUX battery, install another soelnoid after the AUX battery and again continue the lead back to the camper battery. It would be great to hear some opinons and reason on the best way to accomplish isolating these batteries from each other.
Replies:
Posted By: oldspark
Date Posted: August 13, 2010 at 5:57 AM
Another relay. Simple.
My system connects the batteries when the alternator is charging.
The charge lamp "off" actuates a relay that connects the batteries.
Or that relay powers other relays (if many relays or BIG relays that might strain the alternator's charge-lamp circuit).
One relay for each aux battery (the main battery is untouched).
Preferably each relay/battery having its own fused cable - one fuse at the "main" battery end with cable through relay to 2nd fuse near the "remote" battery.
Though than fuses are self-resetting circuit breakers (since batteries can have very high inrush currents that soon decrease).
The reason for separate relays is based on NOT having batteries paralleled unless required or being charged.
If you have one of those stupid "smart" isolators, else other voltage sensing relay/isolator, use that to power the new relays for each battery.
Size the relay to suit your needs (usually from 30A to 400A or more depending....) keeping protection (fuse or CBs) and cabling in mind.
Posted By: hawks
Date Posted: August 14, 2010 at 1:25 AM
Thanks for the advice.
I will go that route and add a second solenoid to the system. I know to fuse the main battery (as close as possible to the battery) on lead going to first solenoid and fuse the lead (as close to the battery as possible) coming out the other side of the solenoid. But, what about the lead going from the AUX battery to the second solenoid and the lead coming out the second solenoid to the camper plug feeding the house batteries? Would I fuse the lead (as close to the battery as possible) going from the AUX battery to the second solenoid and the lead coming off of the second solenoid going to the cmper plug? What size fuses or circuit breakers would be best to use?
Posted By: oldspark
Date Posted: August 14, 2010 at 2:37 AM
The basic aim is no NOT have batteries connected in parallel when not being used or charged.
As to connection protection, the principle is to protect the supply, or cables from the supply. IE - cables from batteries should be protected (fused etc) as close to the battery as practical, hence if that cable shorts, the fuse blows first.
That should answer your fusing question keeping in mind that there need to be fuses at each (or every) end in multiple battery installations.
For such cases, the cable rating must exceed that of the fuse/breaker that is protecting it.
I think you need to add TWO relays to your system, but that depends on what controls that first relay.
If it is a voltage sensing switch, then it needs to be disconnected from the aux batteries - instead it is used to energise the relays/solenoids for each auxiliary battery.
Other systems may not need that sort of rewiring - eg, if IGN or ACC turns on the relay, then it should handle a 2nd relay.
For charge-lamp controlled circuits, I advise a "master" relay that then turns on as many downstream relays as required (without stressing the charge-lamp circuit).
As to relay ratings - that depends on what you want.
You need to supply the charging current plus any additional load.
EG - an audio system system battery may require 200-400A.
My spare ~40AH battery only needs to charge which is about 1-2A when full, but can be as much as 30A or more if partially discharged.
Then there is another 6-10A if I run a fridge etc off it.
I use a 60A relay with 50A circuit breakers at each end.
My old 40A blade fuses used to blow.
Maybe the 50A breakers trip, but they eventually stay on once the current drops after some recharging.
With the breakers I have yet to find a flat aux battery and hot fridge, unlike the 40A fuses where I'd find out 6 hours after the start of the trip!
Posted By: hawks
Date Posted: August 14, 2010 at 12:57 PM
I was thinking of going with using a IGN/ACC on wire to energize the solenoids.
I guess if I gave you more information about my truck setup you could better answer my question on ratings for the solenoids and fuses/circuit breakers.
I need to get new batteries at the same time I rewire this thing. Right now I have a 140 AMP alternator and my main battery as expected is for starting duty only and the AUX battery runs everything else on the truck ie... fuse panels, audio system with two 600 watt amps and a video system. The lead off the 2nd solenoid I will install will run to the camper plug mounted in the bed for keeping the house batteries charged when traveling.
Posted By: oldspark
Date Posted: August 14, 2010 at 8:37 PM
It sounds like 30A is enough for the camper battery.
The aux sound system needs whatever size it draws - eg, 2x600W (RMS!!) = 1200W => ~120A plus the rest - say a 150A or 200A distribution.
(If it's not RMS, then probably 60A or less for the 2x600W.)
I'd recommend using the charge light method (else a voltage sensing isolator) so that your sound system does NOT flatten your main battery if left on, or blow fuses during cranking.
The charge-light relay or voltage sensing relay then powers your battery relays - one of (say) 30A to the camper battery, and a 150A (or 200A or 100A etc) to the audio battery.
And circuit breakers (else fuses) and +12V power wiring to suit. I'd try to size the cable so that there is no more than 0.5V drop at full load - but that's my preference....
The GND return though the chassis should be sufficient, though a GND wire is required for trailers or similarly floating cabins, trays etc.
And don't forget to match your ground wires - ie, alternator/engine to chassis/body and battery- cable(s) at least the size of the+12V cables (else fuses) combined.
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