Renogy has a great support system; contact them with the amount of load you expect to want.
This is where @Taco-Surfer important note comes into play. How I sized mine was by finding out the continuous current available from my Alternator. From there, I could see that I had room for the size charger that would make sense in my application. In my case, I had ample room to add a 40A charger. In hindsight I could’ve gone with a smaller 20A which would suffice in my case.
The key is making sure you leave some headroom on the alternator; then fuse/breaker out your DC/DC connection accordingly. I started with a 100A; but since I opted to connect my winch to the same fuse I learned quickly that it draws alot. So don’t do that.
I was too lazy to get your truck specs; but a quick google search for a mid 2000’s tacoma is as follows:
How many amps is a Tacoma alternator?
Performance
| Mounting Type | Direct fit to OEM Mounting |
|---|---|
| Hot Idle Amperage @ 200F | 170 |
| Max Amperage | 270 |
| Hot Max Amperage @ 200F | 240 |
| Positive Output Stud Size | 8MM x 1.25 Copper |
So personally in your case I would go for a 20A if you’re charging a 100aH battery. That should give your alternator some headroom so it’s not working its but off when running/charging
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The other thing to keep in mind, once you figure out how much headroom your alternator has as @dignon pointed out, is your charging needs.
I only have a 100AH house battery. Assuming I drained that all the way down, which I never do, I could charge that back to full in 4 hours with the 25A redarc system that I went with. If you had a 300AH house battery that you planned to drain 100% on a regular basis then you would probably want to be able to charge that at a higher rate and would want a higher throughput system.
Battery type comes into play a little bit there as well. I’ve got lithium and I know lithium can charge at very high amperages( c-rate ). A quick google search shows that AGM can handle much less and this might then be your limiting factor on what size DC-DC charger you get. If your battery can only charge at 10 - 15 amps then there’s obviously no need for the 50A charger.
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Excellent info, thanks everyone.
Check out my battery heater thread I have going too. I’m in the process of troubleshooting a solution to get my Self Heater to trigger on Shore power. Turning out to be much more complicated that just hooking up a charger.