I am getting ready to install the Keyline 140A battery isolator to charge my 225Ah battery bank. After I ordered it, I read something that said you should only use an isolator that allows half the amount of your battery bank. So is my 225Ah battery bank too small for this isolator?? Is this a real concern?
Battery Isolator size
- Thread starter jennowhat
- Start date
-
- Tags
- battery charging isolator
autostaretx
Erratic Member
No, it's not a concern.
I would guess that the recommendation you were seeing was suggesting a *lowest* limit .. so that would put a lower limit of 112 amps for your pack. 140 is gooder.
An isolator (of the relay type) is only a copper bar, and the amp rating is how BIG the bar is.
There's no such thing as "too big" .... there's only "too small", wherein the current flows melt parts of the isolator.
It's the fuse's job to limit the currents. It's the authorized melting bit.
--dick
I would guess that the recommendation you were seeing was suggesting a *lowest* limit .. so that would put a lower limit of 112 amps for your pack. 140 is gooder.
An isolator (of the relay type) is only a copper bar, and the amp rating is how BIG the bar is.
There's no such thing as "too big" .... there's only "too small", wherein the current flows melt parts of the isolator.
It's the fuse's job to limit the currents. It's the authorized melting bit.
--dick
hein
Van Guru
The fuses protect the wire/cable so best practice is to have one near each battery. With an isolator like this it is conceivable that more current can be demanded from the alternator than it might be comfortable delivering. This could occur if the house battery is significantly discharged or there is a large active load on the house system while the engine is running and the isolator is actively passing current. This is why some folks use DC to DC chargers which can limit the current and allow each system to exist at a different voltage. You should be fine as long as you don't have too many occurrences of the previously mentioned conditions.
All the best,
Hein
All the best,
Hein
Last edited:
Midwestdrifter
Engineer In Residence
I would only skip dual fusing is the run was very short and VERY well protected from shorts etc. Otherwise both ends needs a fuse. Electrical fires are not fun, and will destroy the vehicle...
Thanks, all. Sounds like fuses are the way to go for peace of mind. Are two of these a good choice?
https://www.amazon.com/gp/product/B01N9M4OI1/ref=ox_sc_act_title_1?smid=A130UJC6EIIOR0&psc=1
https://www.amazon.com/gp/product/B01N9M4OI1/ref=ox_sc_act_title_1?smid=A130UJC6EIIOR0&psc=1
Midwestdrifter
Engineer In Residence
No. It is very hard to terminate them properly.
I suggest this fuse holder. Use proper lugs on the cable ends. I also suggest a better quality fuse, as the cheap ones do not perform well.
https://amzn.to/2SGSgnf
Some folks like these terminal fuse blocks, as they reduce total connections.
https://amzn.to/2JERuTq
Last edited:
Bob Laps
Active member
One thing to consider is with the terminal fuse block vs the ANL fuse is u might have better luck tracking down replacement ANL fuses at ur local stereo shop if needed....im no electrical connoisseur but u might only be able to get the other ones thru Blue Sea...I'm not sure but worth looking into
Bob
Bob
One thing that confuses me about this KeyLine kit is it comes with 20' of red 7 AWG cable and 2' of black 4 AWG cable. Does that positive cable seem undersized for the amperage this is likely to see?
I am an electrical idiot so please excuse me if this is a dumb question.
I am an electrical idiot so please excuse me if this is a dumb question.
Midwestdrifter
Engineer In Residence
Don't waste your time with a kit like that. Buy the parts and cable as you need. Pickup a hammer crimper, and make your own cables. There is few good, and many bad reasons for going with a smaller positive run.
Most DC wire runs are sized well above the safety (ampacity) limit. Instead they should be chosen based on max voltage drop. Then fuse appropriately.
Most DC wire runs are sized well above the safety (ampacity) limit. Instead they should be chosen based on max voltage drop. Then fuse appropriately.
Would you mind expanding on that a bit? I found this site for calculating voltage drop. Is the target for max. voltage drop 3% or some other value? I've seen 100 amp fuses used in this type of application but am not sure how that was determined.
Thanks
Midwestdrifter
Engineer In Residence
This calculator is helpful.
http://circuitwizard.bluesea.com/
10% max drop at expected load. Don't forget to include all wiring in the circuit. Neg and positive. For Critical items, like fridges, sensitive electronics, charger feeds etc, aim for 3% or less. Remember that the wire is only part of the equation, so every fuse, terminal, and splice adds some voltage drop. So I try to aim for 2% in the wiring for things like DC fridges, and chargers. Things like LED lights, water pumps, USB chargers, etc are fine with 10%, but I usually aim for 7% or less.
The fuse size is determind by two factors. The max fuse size is determined by the smallest wire protected by that circuit. Wire has insulation with a temp limit, usually 75C or 90C. Then using the gauge to determine ampacity for that wire. This is the current limit for the wire, before the insulation will overheat and fail. For 10 gauge with 90C insulation, that is 40A. The minimum size for a fuse is the limit at which it will nuisance trip. For example a fridge only uses 4-5A continuous, but it will surge briefly at startup. So a 15A fuse is normally suggested. It is good to go larger than the minimum size, as this will reduce the voltage drop across the fuse while still protecting the wire.
http://circuitwizard.bluesea.com/
10% max drop at expected load. Don't forget to include all wiring in the circuit. Neg and positive. For Critical items, like fridges, sensitive electronics, charger feeds etc, aim for 3% or less. Remember that the wire is only part of the equation, so every fuse, terminal, and splice adds some voltage drop. So I try to aim for 2% in the wiring for things like DC fridges, and chargers. Things like LED lights, water pumps, USB chargers, etc are fine with 10%, but I usually aim for 7% or less.
The fuse size is determind by two factors. The max fuse size is determined by the smallest wire protected by that circuit. Wire has insulation with a temp limit, usually 75C or 90C. Then using the gauge to determine ampacity for that wire. This is the current limit for the wire, before the insulation will overheat and fail. For 10 gauge with 90C insulation, that is 40A. The minimum size for a fuse is the limit at which it will nuisance trip. For example a fridge only uses 4-5A continuous, but it will surge briefly at startup. So a 15A fuse is normally suggested. It is good to go larger than the minimum size, as this will reduce the voltage drop across the fuse while still protecting the wire.