second 12 volt battery - 2020 edition

That strikes me as a practical approach and is probably the one I'd follow. The key is having the jump pack so that even if there is an issue, you'll be able to take it in stride and not be much inconvenienced by it, If your experience leads you to add more capacity, you can do that at any future time.

You should note that the car's 12V sockets are off when the car is off, so if you think you ever will want to let the Dometic run with the car off, you will want to direct wire (and appropriately fuse).



In that case, you will be counting on the Dometic's low-voltage cutout to leave you with adequate starting charge if you are longer returning to the car than you planned. The "VCM-06" module in my arrangement above has a similar function, with a fixed shutoff point of 11.8 volts, which always leaves plenty to start the car with no trouble.

My original battery made it to ten years (disclosure: I only had this rig on it for the last four), so the occasional discharge to that point wouldn't seem to have sacrificed longevity much. If it were a regular occurrence that could be different.

Naturally if you will be trusting food that you keep in there, you will want some way of knowing whether the Dometic shut off while you were away, and for how long it was off, and what temperature it reached. I'm not aware of how they handle that, It seems they make some mention of logging data to your phone, but I don't know if they include some kind of backup battery so it would continue to log temperature while shut off. If they're not doing that yet, I guess I've just given away another idea....

 

Well, my thermoelectric cooler draws something like 50 watts. That's why people prefer things like the Dometic, averaging 12 or 13 watts over time to keep your stuff chilled. You've got about 540 watthours total of energy in a (newish, fully charged) battery: 45 amphours times 12 volts (nominally). At twelve watts the Dometic could run for a decent time if it had to.

Adding a 1000 watt inverter by itself won't change much. Plugging stuff into the inverter will. If you really plan to plug in a thousand watts worth of stuff, well, that's equivalent to running about eighty Dometic coolers, right? That won't hold up for as long.

 

A 12v fridge draws significantly more current than 1A. I was skeptical about the current so I consulted the manual, yes no one ever does that anymore. The fridge pulls 7.5 amperes at 12v so it’s easy enough to just buy an in-line fuse holder, 110 amp fuse, pair of power leads with quick disconnects and pair of battery lock terminals that allows you to use pair of cable for each. Won’t cost more than $50 to rig this up on your own. Not sure how you all went with solar and inverters.

 

When they specify 0.98 amphours per hour, they are talking about the average consumption of the fridge while it is keeping your food cool. That will be more than 0.98 amps when the compressor is running, and close to zero amps while it is cycled off. They specify the overall average (and under what conditions, 90 °F outside the box and 39 °F inside) so you can divide it into your battery capacity and know how long it can keep your food cool before the battery is drained.

Yes, for choosing wire and fuse size, you need to know the actual amp draw when the compressor is running.

 

Jerry, I've added a second 12V battery to my 2010 Prius. The link is in my signature below.

 

I have no experience with the Dometic battery, but that's a lot of money for a 40Ah battery.

 

Have you considered something like these jump packs?

Audew 2000A Peak 20000mAh Jump Starter(Former: 1500A)

I believe two of those would theoretically give you the same 40Ah capacity.

 

Does anyone know what the charging profile is like on the factory DC-DC charger (that charges the standard 12v battery from the HV system when the car is "ready") is it just a 14.x volt constant power supply or does it ramp current/voltage up+down like a 'real' charger?

Id like to get rid of the poxy little 12v battery completely and replace it with a much bigger lithium pack if the standard charging system will cope, I have solar on the roof and my solar controller is lithium capable but I can't find any info on the factory charger.

 

Use caution interpreting the advertised milliamphour figures for lithium jump packs. There seems to be a common, ahem, "convention" used when publishing those numbers.

For example, here's the label on the back of my little one:



As far as I can tell, you don't get shown the "29.6 Wh" number anywhere else: not online at their web site, not in the booklet that comes with it, just in that one place, on the label.

Online, the advertising for this unit is "8000 mAh".

This is where the alert reader does a little bit of mental arithmetic and says "hey, wait a sec, 8000 mAh is eight amphours, eight amphours times twelve volts would be 96 watthours, not 29.6. What gives?"

What gives is, a Li-ion cell isn't twelve volts, it's 3.7. To make a jump pack, they stack up four such cells, producing a nice car-jumpable 14.8 nominal volts.

Then they also take the amphour ratings of their four 3.7V 2000 mAh cells, and add those together too, to come up with the advertised "8000 mAh".

Not so fast, though. When you go combining cells, you get to add together the voltages, or add together the currents, but not both. These guys put the cells in series, so the voltages add up, making 14.8. But a series string of four 2000 mAh cells is still 2000 mAh, not 8000. And 2000 mAh times 14.8 V is 29.6 Wh, the honest number on the tiny label on the back.

In other words, the advertising for jump packs is totally sketchy, but it's not just this one manufacturer, they somehow all agreed on doing it this way. Or the first one to market did, and the others had to copy to compete.

When you know that, suddenly it's not so surprising that these might be 25% the cost of a Dometic battery that honestly gives its amphour rating at rated voltage. After all, these are 25% of the battery.

 

Thanks for explaining it all here. I also had noticed that there must be some interesting "convention" (LOL) being applied here with the specs, but not having one of these devices, was not able to get down to the answer as you have done.

What you show here makes exact perfect sense, and I am sure it is right. And yes, literally 100% of everything else on the internet about this is wrong (but not according to "convention"). And hardly anybody knows or understands.

BTW, when I first noticed the discrepancy, I wondered whether they were citing the mAh capacity at a 5V level for the USB power pack functionality. But no, that did not look right either, so I was still wondering what they were doing.

LOL on the convention - reminds me of how the computer hard drive manufacturers eventually got sued class action style for pretending not to know the difference between a GB and a billion bytes, etc. Of course, this battery convention scam is far more distorted than that.

 

On the other hand, after venting the obligatory indignation at the advertising, even a 29.6 watthour thingy that sits in my glove box, fits in my hand, and can start my car (even somebody's old decrepit van one time) is still pretty cool.

It's just a matter to be aware of, when comparing the numbers to something else.

 

The Prius battery was designed to keep power to the computers, unlock the doors, etc.,
and tell the hybrid system everything is "okay" for it to start. Then the hybrid battery charges
the 12V battery, and runs all the 12v items. You don't need a high amperage battery for that.
I don't believe it was designed for the heavy load of a refrigerator.
A second battery, that will handle that load would be the best idea. I don't completely understand
all the "load" details, I just know you need the right equipment for the job.
You can get them for $100-150. And they do a better job than the Prius battery.