TheSamba.com :: View topic - Renogy 40A DC to DC charger: too much for the stock alternator?

Thu Mar 19, 2020 10:28 am

I ordered 4 x LiFePO4 135ah battery cells that I plan to fit under the driver seat.
I'll charge them with my solar panel (probably with an Electrodacus charger/BMS), and I also want to use a DC to DC charger to charge from the alternator while driving.

I am considering the Renogy 40A DC DC charger:
https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger

LiFePO4 cells pull almost 100% of the charging current you can provide them. If I discharge my battery down to 20%, that means I'll be pulling a constant 40A from the alternator for almost 3 hours.
Probably more since I read DC DC chargers are not very efficient.

Do you think it can be too much for the stock 90A alternator? Should I choose the 20A version of the Renogy charger to stay on the safe side and avoid overheating the alternator?

Thu Mar 19, 2020 3:32 pm

I am by no means knowledgeable in this area, but my first thought when I read your post was; what about the main/starting battery? Is there some kind of circuitry in the system that assures the main battery gets charged before that monster gobbles up all the juice?

Hans

Thu Mar 19, 2020 4:30 pm

Personally I'd feel fine allowing 40 amps - but that is just me. Yeah maybe you have to replace your alternator more often but it would be worth it to me.

So 135ah under the drivers seat. Keep us posted on that. I thought max was 120 right now.

Thu Mar 19, 2020 5:06 pm

It may be depending on what else you are running. I think the stock VW one is 90amps.

I am running a subaru w/ a 130amp alternator out of a 3.6L, (23700AA620 is the part number) with a 150AH or so LifeP, and if I'm running the AC, fridge, and lights it drops the charging voltage from 14.5 at the engine batt, down to about 13.1 while the house LifePo is pulling in at 13.6V...so @90 amps output at the alternator with a full load from other devices and charging a LifePo is probably about the max.

Thu Mar 19, 2020 5:11 pm

I asked Renogy this same question and their reply focused on the capabilities of the battery being charged. They advised me to check the battery manufacturer's recommended max charge current. I suspect you will be fine with this setup. Will Prouse thinks so too here: https://www.youtube.com/watch?v=EcVjPap9dkY&feature=youtu.be&t=7

Thu Mar 19, 2020 7:13 pm

I have a Sterling DC to DC 30A charger attached to my 100AH BattleBorn lithium battery. Sterling told me that 14.8 V was the max charge my battery could take and that the Sterling would limit the voltage to 14.4V, if that's any help...

Thu Mar 19, 2020 7:37 pm

What BMS are you planning to use? Could you post the specs of both the cells and BMS please.

Thu Mar 19, 2020 10:02 pm

The BattleBorn 100AH has its own BMS. I also use a Victron BMV-712 battery monitor that I can keep track of via bluetooth on my phone. Super convenient and you can track amp hours used/remaining in real time.

Thu Mar 19, 2020 10:06 pm

What are the full specs of the Battleborn BMS?

Thu Mar 19, 2020 10:11 pm

https://battlebornbatteries.com

Sat Mar 21, 2020 11:05 am

Thanks everyone. Always nice to get answers so quickly, though I'm still not sure what I will do.

shagginwagon83 wrote: So 135ah under the drivers seat. Keep us posted on that. I thought max was 120 right now.

These are the cells I ordered (the 135ah version):
https://www.aliexpress.com/item/4000316031708.html

They're even a little shorter then the 120ah we see everywhere (height is 155mm vs 166mm for the 120ah).

When (if) I receive them, I'll post here to confirm they really fit under the seat.

I even found 160ah cells that should fit on Alibaba, but the very slow communication with the seller was not making me feel confident I would ever receive them. Here's the links for someone more patient or willing to take the risk:
https://french.alibaba.com/product-detail/3-2v-160...67648.html

Sat Mar 21, 2020 11:37 am

dgbeatty wrote: What BMS are you planning to use? Could you post the specs of both the cells and BMS please.

I plan to use an Electrodacus SBMS0: http://www.electrodacus.com/
I posted a link to the cells above. But I'm not worried about the cells (40A is only 0.3C for 135ah) as much as overheating the alternator.

The SBMS0 does cell balancing, low temperature cutoff (to avoid charging below 0C) and can handle up to 30kW (no charing/discharing current flows through it, it just remote controls charging and load devices).

I'll use 1 DSSR20 unit as a solar controller.

If I choose the 20A Renogy, I could also use a DSSR20 to disconnect my LiFePO4 battery from the Renogy when temperature drops close to 0C. That would also give me a double overcharging protection: the Renogy's and the SBMS0's monitoring capabilities, in case something goes wrong with one of these units (I tend to trust the Electrodacus a bit more, but that's just me).

If I go with the 40A Renogy, I cannot use a single DSSR20 to control its input. I could probably use 2 in parallel, buts they're 36 $CAD each - sounds a bit expensive to switch the Renogy on and off. The Renogy has a remote control connection, but it needs to be connected to the ignition so that it only turns on when ignition is on.
But I want it to run only when both ignition is on AND the SBMS0 tells it so (again, for low temperature cutoff).
I guess I could connect the ignition wire through a small relay controlled by the SBMS0. Or maybe an optocoupler, but I'll have to read a bit about how they work.

Sat Mar 21, 2020 11:42 am

How much was the 135ah total shipped? Also how long to take to get in?

Sat Mar 21, 2020 12:03 pm

They give the price, shipping costs and estimated delivery time on the page, nothing was added at checkout.

I paid 764 $CAD shipped. It's supposed to include all taxes and import duties.

I ordered on March 16th, and they say I should receive them before April 25th.

Will Prowse has links to this seller for CALB cells he has bought from them. That kind of makes me a little more confident:
https://www.mobile-solarpower.com/raw-lifepo4-deals-page.html

Thu Jun 16, 2022 8:45 pm

What year of Vanagon? The Aircoolds are 55A stock, so I wouldn't do it on a stock AC alternator. The early watercooled alternator was 65A unless you had air conditioning, then it was 90A. The later watercooled (unsure which year) were all 90A.

As long as you don't have too many accessories on even the 65A should be OK to offer 40A charging and still keep the engine going.

Thu Jun 16, 2022 9:00 pm

I have the Redarc 40 amp DC-DC charger, but I also have a 280 A/h LiFePo4 battery. My thoughts about the OP's situation is that a 40 amp DC-DC charger is too much for a 135 A/H battery. The 25 amp Redarc unit would be a better fit. The really high-end batteries like the Battleborn can take fast charging rates without degrading their life expectancy, but the prismatic batteries are better mated with a charger that charges at around 0.2C. With a 135 AH battery, 0.2C is 27 amps. The 25 amp unit will be easier on your alternator.

One data point I can add is that my 40 amp Redarc DC-DC charger actually puts out 43 amps, although rated to put out 40. Go figure.

Fri Jun 17, 2022 5:54 am

Howesight wrote:
One data point I can add is that my 40 amp Redarc DC-DC charger actually puts out 43 amps, although rated to put out 40. Go figure.

Well, this is interesting. My 40 amp Renogy puts out 33-ish amps for my 150aH battery. Beefy cable size too. 125a Subaru alternator.

The Renogy DC-DC/MPPT 50a model is sounding more attractive because currently when I am on the road, my Victron MPPT turns off because it is detecting a high voltage on the battery.

Fri Jun 17, 2022 5:58 am

shagginwagon83 wrote:

The Renogy DC-DC/MPPT 50a model is sounding more attractive because currently when I am on the road, my Victron MPPT turns off because it is detecting a high voltage on the battery.

Why is this an issue for you? It is supposed to do that.

I like it because it means that when the engine is running my Victron 30a DC/DC charger is doing all the charging. When the engine is off the Victron MPPT solar charger kicks in. Best of both worlds to me.