Adding a big deep cycle battery for 1000W inverter

I have a Xantrex ProWatt SW 1000 inverter that I can currently plug in to either my 2004 Prius electrical system or a big Kirkland Marine Deep Cycle battery. I'm interested in temporarily combining all three into one system so that I can draw 1000W for extended amounts of time, to run tungsten lights for video shoots outdoors. I am just starting to research this, and realizing that I might need a battery isolator to avoid having current from my marine battery damaging the Prius battery since they are such different sizes and will charge at different rates.

If anyone has done this before, I'm interested in what kind of battery isolator you used: brand and type (relay or diode). Or perhaps you got away without a battery isolator?

I'm hoping that I can do this without adding any more wiring to the vehicle; I've currently got a 4gauge 175A Anderson connector on each of the 3 devices (Prius, Inverter, and Deep Cycle battery) and it would be nice if I could just temporarily plug a three-anderson-connector box between all three devices to join them, rather than installing something between the Prius 12V battery and the Prius charging system as most of the battery isolators seem to be designed for.

 

After thinking about it a bit more, my current plan is not to use an isolator. Since this is only something that I will do once in a while and I can be dilligent about not draining batteries down by monitoring voltage while in use, I'm thinking I'll be OK without an isolator.

My plan is:
1. Power up the prius to Ready mode with nothing other than the normal little Prius 12V battery connected.
2. Once the car is in Ready, plug the anderson connector in which connects the + and - terminals of the Marine Deep Cycle battery and Xantrex inverter to the Prius 12V battery, so that everything is on the same circuit.
3. Use up to 1000 Watts from the inverter, which will be stiffened by the Marine battery in case the Prius can't deliver the full 1000 watts.
4. When I'm done, first unplug the anderson connector so the Inverter/Marine battery are no longer connected to the Prius, and then:
5. Turn off the Prius.

The idea is that this is what a relay-based battery isolator would do and I'm just doing it manually.

As long as I start with a fully charged Marine battery and don't drain it too far, I shouldn't have any problems with the Prius starting 12V battery getting depleted, even if I accidentally left it connected after powering the Prius off

Right?

There could be sparking at the Anderson connector when making or breaking connections. But if all the batteries and Prius are at ~12-14V volts, not too much current could flow, right?

What I'm wondering is whether it might be best to make/break the connections when the Prius is off, or to make/break the connections in Ready Mode as described above. I suppose it probably doesn't make much of a difference so long as I don't delay between powering on/off and making/breaking the Anderson connections.

 

Hmm, I just found this battery isolator that seems like it might make this setup more failsafe without needing me to wire it between the the "alternator" (DC/DC) and the little prius 12V battery like most battery isolators require:

12V 150 amp DC battery

isolator and smart split charge relay
for RV, car, and truck
applications

Or this which has a better housing:

JayCorp Technologies - Dual Battery Smart Isolator 12V Relay

 

I just did some experiments with the battery, half drained, attached while connected to the Prius powering a 1000W light through the inverter. In that case I was seeing current of 76A at ~12.3V on the inverter, the battery measured 12.7V, and the car cigarette lighter was around 14V. Afterwards, everything disconnected, the Marine battery was at 12.4V, and the Prius cigarette lighter measured 12.5V. This makes me think that the battery isolators which cut out at 12.6 or 12.7V might cut out on me too much so for reliability's sake I'll just stay with the non-isolated approach.

I don't want to get a higher voltage inverter to wire directly into the HV battery because of the expense, danger, and it's one more piece of equipment to deal with.

I did verify that I can get 1000W without my marine battery, just using the Prius DC/DC, and the Prius battery seemed to be about the same voltage afterwards as when I started the 10 minute test. So that is an option for me, certainly a lot simpler. My reasons for adding the Marine battery are:

- Stiffening the system so that surges don't overstress the Prius DC/DC. E.g. I sometimes use electronic ballast florescent lights which may have surges upon startup and I don't want to blow the expensive 100A fuse in the car.

- Lowering the danger of draining the Prius 12V battery if the DC/DC can't keep up with the 1000W load.

- Keeping the Marine battery topped off in case I want to use it in places where the car won't go.

But given that it sounds like others have successfully drawn 1000W continuous from the Prius without such a stiffening battery, I don't think it's worth putting too much effort into, so I think I'm done with my project now that I've got everything easily connectable with Anderson connectors to 4 gauge wires fused by the batteries.

Thanks.

 

Just to update, I had a power failure last week and used this system to run an electric jigsaw. No isolator, just connecting the inverter and stiffening marine battery with anderson connectors. I decided to plug the marine battery anderson connector into the Prius while the Prius was running in Ready mode, in order to get everything at similar voltages and prevent the possibility of the little Prius battery or inverter being damaged by surges in case the Marine Battery that I pulled off the float charger happened to have gone bad and had internal shorts. Probably way more cautious than needed, but I don't want to have to think through this stuff everytime I do it. I'm curious if anyone knowledgeable can say whether I should keep up this practice for safety, or whether it's safer to plug the marine battery in before starting the Prius.
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