Using an MPPT Charger with a DC/DC Charger of a different brand.

[cplantijn]

Hello ladies and gentlemen, I just bought a 2016 Ford Transit Extended High Roof and am starting my build. I'm taking things slow and building it out as I go camping in it. Right now I'm setting up a primitive electrical system.

• Battery 400AH 12v
  • Link -> https://www.meritsunpower.com/merit...thhth-bms-for-marine-rv-solar-system-golf-car.
  • Price is $1275 USD before shipping FYI.
• DC to DC Charger (charging from Transit's Vehicle Batteries/Alternator)
  • Link -> https://www.renogy.com/12v-60a-dc-t...puKGsDc4Jjs4mNM0J43EpJTA08gBx4sxoCOxEQAvD_BwE

When I add solar, I'm thinking about going with about 900W of solar (3x300 watt panels in parallel). This will reach a maximum output of 75 amps. I plan on using a Victron MPPT Solar Charger. (I know it will be much lower due to efficiency and real world conditions. I live in Florida btw).

So, I'm thinking, hypothetically when I drive I may push up to (75 amps (solar) + 60 amps (DC->DC)) = 135 amps. I hear that the recomemnded charge rate of LiFePO4 batteries are ~0.2C, so that's (400 * 0.2) 80 amps.

So, that's the setup. Here's the question. How can I make sure my overall charge that goes into the battery does not exceed 80 amps? I know that some companies offer a complete package MPPT/DC to DC unit, but the highest amperage one I saw was a Renogy for 50 amps. Assuming I have multiple systems from different brands, how can I limit the charge amperage going into the batteries?

What if I connect a second 400AH in parallel to make a 12v/800AH system? Does that increase my maximum charging current to 160 amps or does it remain 80 amps? Thanks in advance to any replies.

 

[HRTKD]

Yes, adding a second battery increases the allowed charge amps, as long as both batteries are in sync and taking a charge. Before you get too far along with worrying about the charge rate, verify with the manufacturer what the actual charge rate is. My EVE 280Ah cells are rated for .5C standard charge/discharge rate.

With the Victron components you _may_ be able to control the MPPT and DC-DC from a monitoring device like their Cerbo GX. I don't have the Cerbo GX yet, so I can't give you a definitive answer.

 

[rickst29]

In addition to the factors described by Jim, I'll mention that (1) the Victron is pretty easy to reprogram to a lower output current limit. You just tune it downwards for the driving days, and then tune it back up to maximum when you park for camping.

I do find it weird and concerning that a "400Ah" battery would allow only 80A for either charging or discharging. (Their "300Ah" model quotes only 60A for both figures, while their "100A" allows for 50 and 50 Amps). Maybe they've done a very poor design inside, with "tiny" bus connections and a single "tiny" BMS arranged in a bad way. As a competent homebuilt, an LFP battery of that size would to handle continuous discharge of at least 250A (maybe as much as 400A), and charge at approximately 50% of the discharging limit.

 

[NPhil]

I've edited to reflect an updated opinion:

My reading of the specs _was_ that the battery's built-in BMS will allow a maximum of 80A charging current. Meaning that your equipment may be prepared to "push" more, but it won't happen because that's all the BMS will accept. But, when I checked my assumptions, they were evidently wrong. I guess I still think a BMS should do that, but, I do not advise the reader to assume that it will.

 

[cplantijn]

Yes, adding a second battery increases the allowed charge amps, as long as both batteries are in sync and taking a charge. Before you get too far along with worrying about the charge rate, verify with the manufacturer what the actual charge rate is. My EVE 280Ah cells are rated for .5C standard charge/discharge rate.

With the Victron components you _may_ be able to control the MPPT and DC-DC from a monitoring device like their Cerbo GX. I don't have the Cerbo GX yet, so I can't give you a definitive answer.

I'll look into that Cerbo GX, but what rickst29 said below sounds like a good alternative as well. And for the charge amperage, does adding a second battery add it's charge amps to the total charge amps allowed in the system when batteries both in parallel?

 

[cplantijn]

rickst29 and NPhil, here are the specs in the attached screenshot.

It looks like the battery does accept Max. Charge current of 0.5C and max continues Discharge current at 0.5C. However it also mentions a "Standard" charge current that only 0.2C. So should I look for a LiFePo4 battery that nominally accepts 0.5C charge as well as 0.5C max cont. discharge?

 

[NPhil]

rickst29 and NPhil, here are the specs in the attached screenshot.

It looks like the battery does accept Max. Charge current of 0.5C and max continues Discharge current at 0.5C. However it also mentions a "Standard" charge current that only 0.2C. So should I look for a LiFePo4 battery that nominally accepts 0.5C charge as well as 0.5C max cont. discharge?


View attachment 60146

I'm editing my comment after checking my assumptions, which were, apparently, wrong:

I missed the "Max Charge" line, sorry.

And my assumption, that a BMS is intended to protect the battery from excessive charging current, so currents which are really dangerous to the health of the batteries are limited, was apparently wrong. But it looks like you are still easily within 200A, so my guess is still that your design isn't obviously walking into the danger zone.

BTW, if I wanted to limit charging, I would limit the current originating from the alternator first, not from the solar, because it's being produced, typically inefficiently, by expensive gasoline, except in a few circumstances like descending a long hill where zero throttle isn't keeping you from exceeding the speed you want. Maybe a switch on the dash which you would use only when the solar hadn't been adequate to get your batteries charged?