Adding a second battery

Answering my own question (since the photo's specs don't include this number) the Deka 908DFT battery capacity is 185 amp-hours. 440 amp-hours (conflicting spec sheets).

Based upon John16ct's comment, that would put its maximum draw (for charging) at about 0.2 * 440 = 88 amps.

So, again, #6 wire with 80 amp fusing should be fine.
As long as that's a short 88 amp demand, an 80 amp fuse may be adequate.
Time to revisit the Wizard and see what it says about 90 amps.
Yes, it's happy with 90 amps through 3 feet of #6 ... as long as it's 105 C dry, 75 C (167 F) wet -rated.
(looking at my own current capacity chart, #6 is rated for 80, 95 and 105 amps ... depending upon insulation temperature rating. TW is 80 amps, THW is 95 amps)
A three-foot length would lose about 10 watts, and cause a 0.11 volt drop (at 90 amps).
When you buy your wire, choose some with 105 C temperature rating.
Fuse for 90 amps.

The goal of the fusing is to prevent over-stressing the wiring.
If something weird (tool dropping across live terminals) happened, 80 amps would pop.

Although high capacity, a "typical" flooded lead acid starter-style battery will sulphate if only given low-current loads. Eventually that may (or will) cause lowered performance (lower peak current capabilities, gradual drop in total stored energy). If your shore-powered charger includes such features as de-sulphation sessions, it would help forestall those effects. A DC-to-DC intelligent charger would also help.

--dick

Regular manual "equalizing" sessions at higher voltages after Fully charged, as per the maker's documented protocol

will be much more certainly helpful

than the "high frequency pulsing" desulfation bells and whistles, IMO usually snake oil.

In particular when the situation prevents recharging to 100% Full each cycle as per mfg endAmps spec, aka PSOC

Monthly as a minimum even with no such abuse.

My inverter is magnum sine wave MS-2812 with digital remote, I have to research to see if there are any issues disconnecting shore power. I don’t believe there would be as that is just for charging and since I won’t be using the inverter to charge the house battery this unit is way overkill now. The inverter is only to power up a single 120v outlet towards the back of the van that I use to charge my cordless tool batteries with. I removed my seat and easily found the D+ location, plenty of space to mount the relay also which is nice.

Cabs said:

My inverter is magnum sine wave MS-2812 with digital remote, I have to research to see if there are any issues disconnecting shore power. I don’t believe there would be as that is just for charging and since I won’t be using the inverter to charge the house battery this unit is way overkill now. The inverter is only to power up a single 120v outlet towards the back of the van that I use to charge my cordless tool batteries with. I removed my seat and easily found the D+ location, plenty of space to mount the relay also which is nice.

Click to expand...

I would be confident there is no issue disconnecting the shore power as from what you say it is just acts as a AC power source and charger when connected to shore, an inverter when not. Even running both alternator with magnum on shore should not cause any issues.

With such a large inverter I would be conscious of what load you put on it when the new ignition operated solenoid is operated, especially if the house battery is low, as the load current passed on to your new cables and solenoid could be significant.

Things like realizing the battery is too low to run a load and starting the motor to supplement it would need to consider the load. Remember 2800 watts is over 200 amps from a 12v battery so cable protecting fuses or circuit breakers are no just nice to have.

nice unit

With the ARC version remote / shunt, the charger side will precisely end charging (drop to Float) based directly on endAmps every cycle under widely varying conditions, rather than on its SoC guesstimate or yours of AHT.

After remeasuring everything I will be using 5 ft of wire so I'm going to bump it up to #4 based on the wizard chart. I'm guessing that Amazon is the best place to get fuses / breakers and the wire with terminals

Since you're in St Pete, visit a good marine (i.e. yachts, fishermen) supply house.
They will happily sell, cut and terminate cables to your specification. (choice of colors, too)

That's how i did mine.

With any luck at all, they'll also have a wide range of BlueSea electrical widgets hanging on blister-pack racks.
(like Fisheries Supply, here in Seattle)

--dick

Thought I could tag a question here: I have one 105 aux. in the spot supplied by sprinter. (Engine compartment. It is properly wired through the isolator under the seat. To add single 200 amp battery to boost the capacity, can I cable off the 105 amp battery to hook in?

Trawlercap said:

Thought I could tag a question here: I have one 105 aux. in the spot supplied by sprinter. (Engine compartment. It is properly wired through the isolator under the seat. To add single 200 amp battery to boost the capacity, can I cable off the 105 amp battery to hook in?

Click to expand...

You (physically) can, but you shouldn't. (i.e. there are significant trade-offs)

If the two batteries are "unequal" (such as 105 AH vs 200 AH, and over-a-year-old vs new), then they won't share the charging and the loads "equally" nor "optimally".
(escalate that "won't share" if its a mix of flooded lead acid vs AGM).

The length of cable from the existing battery to the new one will also skew the results.

But: at worst, you'll "simply" shorten the life of one of the batteries, and won't get the full 105+200 AH worth of service from the combined pair. So if that's not a worry, then you can do it. Monitor the fluid levels frequently. (otherwise "at worst" could be a shorted cell due to plate warping, followed by possible fire).

Many threads discuss the down-sides of "mix and UNmatch".

In their equipment guides, MB specifically says to NOT parallel another battery to their Aux battery. But part of that is due to the relay and wiring sizes they're using.

If you ran a DC-to-DC charger between the two, or even had a 2nd isolation relay, ... AND separated the loads (the 200 AH runs the fridge, the 105 AH runs the LEDs) then it would work better.
(if you only had a 2nd isolation relay, they'd still mess up properly distributing the charging currents)

--dick

Unlike batteries in **parallel** for normal use is no problem, so long as the charging profile is within spec for all of them.

They should just be maintained, tested for health etc independently.

autostaretx said:

You (physically) can, but you shouldn't. (i.e. there are significant trade-offs)

If the two batteries are "unequal" (such as 105 AH vs 200 AH, and over-a-year-old vs new), then they won't share the charging and the loads "equally" nor "optimally".
(escalate that "won't share" if its a mix of flooded lead acid vs AGM).

The length of cable from the existing battery to the new one will also skew the results.

But: at worst, you'll "simply" shorten the life of one of the batteries, and won't get the full 105+200 AH worth of service from the combined pair. So if that's not a worry, then you can do it. Monitor the fluid levels frequently. (otherwise "at worst" could be a shorted cell due to plate warping, followed by possible fire).

Many threads discuss the down-sides of "mix and UNmatch".

In their equipment guides, MB specifically says to NOT parallel another battery to their Aux battery. But part of that is due to the relay and wiring sizes they're using.

If you ran a DC-to-DC charger between the two, or even had a 2nd isolation relay, ... AND separated the loads (the 200 AH runs the fridge, the 105 AH runs the LEDs) then it would work better.
(if you only had a 2nd isolation relay, they'd still mess up properly distributing the charging currents)

--dick

Click to expand...

Good stuff, really appreciate it!

After finishing the install I only have 2 regrets. The first one is I should have done this a lot sooner it was very simple to do. Second I should have taken a few pics as evidence lol. I mounted the relay under the seat near the D+ terminal strip which in my case was perfect. Thanks for all the help. Now who has added flush mount USB chargers in the dash?