I'm wondering something which is really at the root of the "possible" aspect of Tom's original question: If the SOC (as measured/calculated by Ford's system) drops below their chosen minimum (40%?), will the vehicle not even attempt to start? Also, at what level is it likely that the vehicle actually couldn't start from it's own battery?

Tom, another item making a battery to battery recharge of your deep cycle pretty inadvisable: the NiMH batteries self-discharge characteristics. That traction battery may loose as much charge just sitting there, as your lead-acid battery gives you to use over the same period of time!

 

Sorry, I'm not following..........

 

A better and much more efficient method would be to use a "hobby" multichemistry battery charger running off your FEH's 12v to charge the other battery.

This way, you're not dumping power in the power conversion from 12vdc to 120vac then back down to ~15v.

On the subject of self-discharge, this rate is quite low. So long as the FEH is run once a month or so, it should be fine. Several years ago, I was deployed for 6 months and my FEH sat in a lot. When I returned, not only was the traction battery essentially dead (<<40% maybe?) but the lead acid was dead as well. My theory is that the lead acid provides control power for the power electronics in the FEH but I can't confirm this.

 

Your theory is correct, most of the time a really good low leakage capacitor bank would suffice (once daily "start") as an HSD "battery"

You CAN NOT fully charge, "top off" the charge level of a DEEP CYCLE battery via connecting in parallel with the battery in an automotive battery charging circuit. Exceptionally so, possibly, with a hybrid battery that is NOT expected to provide the power, enough POWER, required to turn over an engine using a 12 volt starter motor.

In point of fact these days in order to extend battery life very few automotive charging systems are designed to charge the battery above/beyond the level needed to provide at least one extended period of starter motor operation. With the advent of virtually INSTANT engine starting due to the adoption of DBW, EFI precision, coil-on-plug ignition, and lightweight reduction geared starter motors, battery starting POWER capability is simply no longer a major issue.

On the other hand the "host" battery charge level, voltage charging level is no longer high enough for the ops needs.

With the inverter approach one of these new multi-stage charger can be used, assuring that the "auxillary" deep cycle battery will be fully Charged.

And yes, you will lose a small percentage, maybe as low as 5%, of the charge power in the process, but so what, you end up with a fully charged deep cycle battery that you can use to extend your dry-camp period for days between charges, not just hour to hour.

 

Your average modern multichemistry charger uses a multistage algorithm and they're ~90% efficient. More than 5% is lost by using an inverter/charger combo. Fact is, most of these devices are cheap and horribly inefficient. As said earlier, you can only get ~150Wh from the HV battery so the OP needs to use the most efficient method available if he wants this to even have a chance of working.

The reason you don't want to use a starting battery to run a charger is the fact that they actually have very little capacity. It has nothing to do with how they're charged. They're simply not designed for deep discharges.

 

"..they actually have very little capacity..

Methinks you're confusing the HSD "starting" battery with a more standard automotive starting battery. The latter typically have a HUGE surplus of capacity for duties other than "starting".

In any case, I have always assumed the HSD 12 volt and HV battery would not likely have enough combined charge level to recharge a "fully" discharged, or mostly discharged, deep cycle battery of any reasonable size for dry camping. So I have made the followon assumption taht somewhere in the process the ICE would fire up automatically once the HV battery charge was depleted to a level that warranted such action.

"...Fact is, most of these devices are cheap and horribly inefficient..."

I would go with inexpensive wording rather than cheap, since cheap connotates more negativity rather than just price.

"...horribly inefficent..."

NOT, Never..!!

Where are you getting this information? Most, if not all, DC-to-AC inverters use solid-state devices and PWM switching and are 90-95% efficient.

What is the efficiency and the current, amperage, limit of the HSD HV-to-12 volt inverter? 90%/20 amps...?

I suspect that most dry-campers will very likely already have a MSW DC-to-AC inverter "on-board". If not a "cheap" 100W device will probably suffice.

Harbor Freight, 66814, 400W $29.99.

 

I seriously doubt a cheap <300w automotive inverter is anywhere near 95% efficient. I would expect 85% at best. Expensive power supplies with power factor correction peak around 90% efficient and inverters are similar.

You failed to mention the efficiency of your AC powered SLA charger. A cheap low power AC charger probably uses a laptop style power supply to convert back to DC and those are only ~60% efficient and that isn't even considering the DC-DC converter charging circuit.

No matter how you slice it, multiple power conversions will always be less efficient than a single stage conversion. Even if each stage in your scheme was 90% efficient, it would still be less efficient. Remember that the efficiency of each stage in the power conversion adds up (or more correctly, multiplies).

Personally, I would install a 24vdc charger/inverter/battery maintainer in the trailer and get a Honda 2000i to recharge it and supply AC if/when using larger loads.

HF claims a PEAK of 87% efficiency but I see nothing in the manual. If I'm not mistaken, I recently saw one of these fail in the field to provide the rated power.

 

Alright, that was just conjecture, my point was that the NiMH cell characteristics, for cells used in traction battery applications that I've seen info on, loose charge much differently than lead-acid batteries. However, that would be observed / measured on cells in storage. NiMH are more stable than NiCd, but I guess that's not saying much.

I still feel that connecting the deep-cycle battery to your Escape "12 volt" system could be successful, but would be quite limited without letting the ICE run to generate the power needed.