Questions about trailer solar systems and charging off the tow vehicle, and shore power


Sorry I do not understand the meaning of the former, nor the point of the latter.
Anything on paper would be great see.

As to 20h load testing.....the last one I did at the manufacturers stated rate had the batteries at 11.9v in 7 hours.


Well-known member
Sorry I do not understand the meaning of the former, nor the point of the latter.
Are you aware of any commission info that is in writing?

My comment about the 20h test that latested 7 hours reflected your comment "Ah capacity is rarely as great as maker claims".


My comment about the 20h test that latested 7 hours reflected your comment "Ah capacity is rarely as great as maker claims".
Was that when new? or how old? Which model battery?

Note stopping at such high SoC does not really give any accurate measure of Ah capacity.
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My commissioning blurb:

Breaking in lead batteries properly, including initial "commissioning charging" is critical to getting max longevity, and as noted, will often increase their capacity.

This process for lead banks does involve a very similar process to load testing, and done properly, and infrequently over the bank's lifetime, neither causes any damage.

But yes, capacity will likely further rise after the breaking in period, so do a 20-hour test before and after, and use the higher Ah result as your benchmark for comparison later on.

If not done then, your test must be compared to the mfg rating, which with quality makers in the US, is the 20-hour AH rating, but often not accurate, as MS points out too optimistic.

Obviously the other specs are for a different purposes, so IMO just ignore them.


The first few dozen cycles, go easy, avoid high current rate discharges and charges.

First step, really fully charge, takes 6-8 hours at Absorb Voltage, stop as per mfg spec, or endAmps tapering to .005C (0.5A per 100AH)

Then, manually follow mfg equalizing instructions, if applicable.

Next, discharge at .05C (5A per 100AH) for about 20 hours, but stop discharging before 10.5V.

This discharge cut off voltage is by definition SoC 0%, and much lower than what should be ever be used in normal cycling.

But do not believe anyone claiming such maintenance routines are as "harmful" as the usual routine abuse from user ignorance.

It would be harmful to let the battery sit at such low SoC, so immediately as possible after that 10.5V is reached, start recharging, ideally at a slow rate, ~.15C or lower

Such slow charging should continue for the rest of the break-in period, say two months if regularly cycling, or 50 cycles over time, trying to avoid huge draw currents as well.

load testing for SoH / 20-hour residual capacity

Note this procedure is similar to the above, so that they could be done in conjunction with each other.

This one is just more precise, requires maintaining the load at a constant as possible current rate as SoC and voltage drops.

CTRL-F search for

20 hour capacity

At MS's site here

Better yet, read it all, and check out the rest of the site.

SoCal Tom

FWIW, I’ve monitored charging with a 7 pin connection to my tear drop. As stated above the charge rate is low, it seems to be enough to replenish what my fridge uses, but not enough to recharge the battery from a low level. iIRC last trip I took about 500 watt hours out of the battery, on the 3 hour trip home it put back about 100. My alternator is capable of 200 Amps, so that wasn’t the limiting factor, it was likely the wire.

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Well-known member
my 7 pin plug only contributes ~5A max to the trailer, limited by its wiring.. Ive had a deeply discharged battery and driven for 6h and it barely put a dent in recharging it.

now I have 325W of Fixed Solar on the roof puts out about ~5A too in cloudy conditions, so when towing in overcast it charges with ~10-12A which is better than it was.. when sun is shining its charging @ ~25A and can easily charge over 1kwh in just 3-4h

Tow Vehicle charging never really worked for me, I drop trailer as base camp and then go explore around with my tow vehicle.. even with upgraded wiring and high amp charging capability I simply dont drive far enough each time I hookup the trailer to get it a good full charge.. When I'm trying to get my back to survive weeks on the road I simply cannot drive for hours on end all the time.

For me a GenSet was better option than upgrading my tow vehicle trailer wiring to be a backup for solar.. if sun's not shining I can fireup genset and leave camp with my tow vehicle, come back to everything fully recharged and I didnt need to even listen to it run.. I think Alternator Charging is less for the trailer crowd and more for the CamperVan, RV, Truck Camper crowd that is always carrying the battery bank, even when exploring away from 'base camp'.


For me it's not / either or, best to maximize usage of all available inputs.

Driving should get high amps into a depleted bank, reducing my expensive genset runtime.

But yes, requires investment in infrastructure, each owner must decide what's worth that.

Using existing pins on trailer plugs is IMO only for the lightest of loads, otherwise silly, even dangerous.

An efficient 12V compressor fridge may only draw 4-5A running flat out.

A three-way much higher, why the 12V is only useful while driving.


Well-known member
nothing at all wrong with maximizing your charge sources, I totally get that.. one thing that should be considered is the vehicle its self, for example my tow vehicle already has a sophisticated battery management system in it that requires me to input a new battery serial number into the diagnostics computer so it resets its charge profiles for a fresh battery.. it ages out batteries and will end up undercharging a new battery unless its informed its got a brand new battery.. Tapping off the starter battery under the drivers seat would be easy but likely screw with the built in charger, getting it off the alternator output would probably work with a B2B charger in the trailer but its a unibody SUV not a frame rail truck so getting power from the front to the back would basically require removing a good portion of the interior.

If you've ever owned a German vehicle, you'd probably be weary with mucking around too much with its electronics unless you wanna just put tape over the CEL.. for me with good solar and a genset the need for high output in-tow charging is basically nothing.. It'd be nice yeah, but at this point its gains would not be worth the investment...

IMHO GenSet just makes sense to prioritize investing in before a high power in-tow charging system, Your vehicle engine is more expensive than your genset to operate if you end up idling either at camp to keep your battery bank happy and reduce abuse.. relying on in-tow charging as your primary charge source will be a recipe for battery abuse for most people, its a long drive to get a full absorb to 100% on lead.. but if you want both, then all the power to yeh*

*pun intended.

Recommended books for Overlanding


Yes in that case I'd go B2B for sure, and prolly run the wire underneath.

I wish they made TVs designed to last 50+ years after an EMP blast, but yes they wouldn't get as good mpg


Yes for those whose primary use for a genset is charging the bank,

a DC Generator works well.

Can simply be a powerful alternator for a couple hundred, paired with a right-sized motor.

OTS units can be super expensive, with limited repairability depending on location.

Fischer Panda and Balmar are another two to check out.
Well the adventure continues. Despite being on shore power the voltage is down in the 13 range and there is a steady 2.7 amp draw. Battery monitor says I'm down to about 65% as of this morning. Is the charger going to kick in at some point or is it waiting for me to tell it what to do?

I don't have a clamp meter but I can get one. Probably should have one anyway.


Tail-End Charlie
Don't trust the monitor at this point.

First verify that the Iota is working. Kill power to it and power it back up. It should raise the voltage to high 14's, then after a while it will float. In float it will hold voltage in high 13's and will also supply enough amps to feed that 3a load.

If it is doing that, the battery isn't being drained and the monitor is lying about that 65%.

I don't have time at the moment to lookup the specs for the IQ-AGM to see exactly what the voltage setpoints are. It should kick in a bulk/absorb/float cycle when powered up, after 7 days at float or when the voltage of the battery drops below a threshold, probably 12.8v.

Is the charger neg straight to battery or through shunt?

Lots of clamp amp meters only do AC, if you get one, get one that also does DC. There's a thread around here about that.
I'll power the Iota down and fire it back up tonight.

I have the regular IQ, I went with FLA batteries.

The voltage is still over 13, maybe the monitor is just confused...? It says it's been synchronized once but if I did that I didn't meant to. All the installed components say the factory defaults are right for FLA batteries.

The neg cable off the charger is to the shunt, in fact the shunt is the ONLY thing connected to the Neg. side of the battery. Everything else is on the load side of the shunt. Including the neg side for the solar.

Thanks again for all the help.

And my monitor is the Victron. I think somebody thought I bought the other one that was recommended. In case that matters.
So last night, with the monitor showing 64% charge, the light on the IOTA charger indicated it was in float. I unplugged the shore power, no change. The inverter and monitor both showed 13.7V. I unhooked the bank, and hooked it back up. Suddenly the monitor shows 100% charge. I unhooked the IQ module momentarily and then plugged it back in. It flashed 6 times like it's supposed to (confirming a 12 volt system) and returned immediately to float.

I think the charger is fine and the monitor is confused about the battery's capacity for some reason. I suspect I just haven't figured out how to set the parameters on it.

The monitor still shows a 2.7 amp draw all the time though. It's pretty consistent.