Ok, so I think I'm getting closer to pulling the trigger. Initially I was targeting a 50 - 60AH batter, but now I think I'm going to go with a 100AH battery. I believe this will give me 1280WH (100AH x 12.8V) from reading various threads on here. Assuming a 5WH draw from the A129 Pro Duo and 5WH from a MT1, that I want to install on in the future for my side mirrors, this gives me roughly 128 hours of run time without charging which is roughly 5.33 days.
The only hiccup is the wiring part. The normal add-a-circuit fuses are rated up to only 10A. I searched Amazon and found some that would be good for 20A. I thought this would cover a 18A DC-DC charger, but
@Outbacknomad is saying to use a fuse rating twice the AH rating of the DC-DC charger which would be a 40A fuse and I'm not sure they make a 40A fuse tap or if my fuse panel is even rated up to 40A. The highest amp fuse I see in it is a 30A fuse for the windshield wipers. If I put in a 40A fuse, then that means I should run wire that is able to handle 40A, right? This seems like a lot thicker cable than I had originally planned on running that was rated for 20A.
If I can't find a fuse tap rated for 40A, but my fuse box can handle that, then I'm thinking I might be able to use an empty fuse slot. I would just need to find out which connector corresponds to the empty fuse slot, crimp/solder a contact, and insert it into the connector. However, this assumes that a cable rated for 40A would fit in the connector.
I'm thinking I could figure out where the empty fuse slots go by using a multimeter to do a continuity check on the non-energized side of the empty fuse slot and probing the empty slots on the connectors.
Here's some photos of my fuse panel so you can see some of the empty pin/cable slots on the connectors:
I'm leaning pretty heavy towards ExpertPower's EP12100 100Ah LiFePO4 battery. I searched for a tear down video as suggested by
@SawMaster and found a video by Will Prowse tearing down and testing the EP1200:
He said he doesn't recommended because it failed his low temperature charging cutoff test. However, he posted some emails from ExpertPower where they acknowledged the problem and are working to correct it. I didn't see any updates in the comments as to whether the batteries now have a functional low-temp protection, but at a price point of $560.99 I'm tempted to purchase one anyways. I guess I would have to pay attention more to make sure I wasn't charging it in low temperatures. I did find an interesting comment:
"Awesome review. Thanks. Get some sleep! It looks like they use some grade B/C cells that didn't pass 75Ah mark of the manufacturer, and were sold as sub 75Ah cheaply to them. Then they did capacity matching and put ~110Ah in pairs. No other explanation possible I think. It wouldn't be so heavy otherwise. Rest looks okish."
I'm not really sure how much the cell grade matters if the battery outputs the required wattage. Also, Will mentioned the battery is not capable of charging/discharging at the a full 100Ah, but that's totally fine given my use case.
You can connect it to an accessory fuse that is only powered when the engine is running, but it would be best that the fuse rating is twice the AH rating of the DC-DC charger.
Why does this fuse need to be twice the AH rating of the DC-DC charger?
If the you haven't got a fuse double the power you need. Use a relay so the accessory circuit switches on the relay, and then a power supply from the starter battery via the rely powers the DC-DC charger. Easy.
I'll have to look into this as I'm not familiar with relays. I guess a lot of this hinges on me figuring out the max amps my fuse panel is rated for.
victron energy earlier this year finally released information about the importance of sizing of lifepo4 to alternator for direct charging off it no dc to dc converter needed blowing the myth out the water of alternator damage if done correctly.....
Do you have a link to a video? I found a video where they talk about some alternators can output full loads at max RPMs and when they halved the RPMs the alternator started smoking. Their recommendations were installing an alternator that can handle the loads at low RPMs, fit a regulator and temperature sensor, or use a current limiter or DC/DC charger. Also, they mentioned something about some lithium battery systems disconnecting the battery from the charging source once fully charged potentially damaging the alternator. The recommended a Buck Boost and a DC/DC charger to avoid this:
Charging lithium batteries from an alternator, be that in a boat or vehicle, presents some challenges when compared to the straightforward nature of charging lead based batteries. The video in this blog highlights these lithium challenges and shows you what can happen when things go wrong –...
www.victronenergy.com
a 100ah is 6x6x8 inches and youd be adding 3 times useable capacity of your car battery that charges from flat in 2.5 hours driving///
The ExpertPower one is 13 x 6.8 x 8.4. With an 18A DC-DC charger it seems like it would fully charge from flat with a little over five to six hours of driving which is pretty close to how long I would drive it when I'm using my car to go to work every day. I was thinking I could even just buy a 10Ah battery to swap in if I ever needed to pull the 100Ah one out to charge in the house. I think the 10Ah would be enough to cover the in-house charging time of the 100Ah battery.
Finally, I need a way to secure the battery. I was looking at some battery boxes at Wal-Mart and found one that would fit the ExpertPower battery. It has a strap to secure the case. I noticed in my trunk there's an anchor point to secure a child's car seat. It seems like I could thread the batter box strap through it and it would be fairly secure in the event of an accident. The battery box would also help prevent any inadverent contact with the battery posts.