Two (2) B2B chargers in a vehicle to get double the charging amps?

[Ample]

I have a Renogy 40A B2B charger and it's working well. I also have a heavy-duty alternator (220A if I remember) and thus could run a higher capacity B2B charger.

I'm planning on a 4th 100Ah LiFePO4 battery and thus I wouldn't mind a more powerful B2B charger to charge the soon-to-be bigger bank.

Renogy only makes a 60A charger.

Could I simply add a 2nd 40A B2B charger in parallel?
Could doing that provide 80A of charging?

The 2nd 40A unit could also provide some fault tolerance. If one of them dies, I still have 40A of charging.

I did ask Renogy but their front-line support simply said "No, don't do it" without further explanation.

What problems could I face with 2 B2B chargers in parallel?

I'm not an expert but I know that I've simultaneously run:

1. 60A MPPT charge controller pump out about 25A
2. the 40A Renogy B2B charger (engine on)
3. a 50A power converter (plugged into shore power)

And they all played nicely together. This was only for a short test. Nothing fried.

It was kind of like my 3 Li batteries, which were from 2 different manufacturers. They figured themselves out.

I could see that batteries charge and discharge at different rates. Sometimes at night with no solar and away from shore power and no B2B, 1 or 2 of the batteries would charge the 3rd or sometimes one would charge the other 2 until they equalize.

I'm hoping that two B2B batteries could run simultaneously too. Thoughts?

 

[TacomaJoe]

Two B2B chargers can run together, keeping in mind that they will also double the draw from the alternator.

 

[mikefitz]

Two 40 amp B2B will be OK running in parallel. Note with a 40 amp output, the input current current could approach 45 to 50 amps.
The 220 amp alternator will have a lower output at low engine speed, also the vehicle electronics could use in the region of 50 to 60 amps under some conditions. You should have enough 'spare' current for the two B2B. Idealy monitor the voltage at the starter battery when you are commissioning the chargers. A sigh of the alternator getting 'upset' with be a considerable lowering and possible surges of alternator volts.

Mike

 

[Aussieguy]

I would sell the existing batteries and get 4 matched...
unmatched may work ... but you will always be monitoring them and wondering what was that spike..

are you planning on using more power from the "New" bank or just adding extra battery so you can have cloudy day insurance?

would look at the solar before looking at B2B .. any chance of increasing the panels or their efficiency?
add a ground panel?

Did you take into account the wiring cost for another B2B against an upgrade?

 

[Aussieguy]

grrrr missed it was old thread!

 

[Ample]

I would sell the existing batteries and get 4 matched...
unmatched may work ... but you will always be monitoring them and wondering what was that spike..

are you planning on using more power from the "New" bank or just adding extra battery so you can have cloudy day insurance?

would look at the solar before looking at B2B .. any chance of increasing the panels or their efficiency?
add a ground panel?

Did you take into account the wiring cost for another B2B against an upgrade?

It's for an RV used for travel, i.e., at most a few months out of the year. So I'm not super fussed about it.
I have power meters on each battery and I see they are behaving well. Certainly well enough for me to not worry about it.

Re: adding another battery. I just need extra capacity. I had been getting by with just ONE Li battery. 100A of continuous power is awesome! I love the LiFePO4 technology!

With my current 3 batteries, I can theoretically have 300A of power and my cabling can handle it. But realistically I only need about 125A max and only rarely (microwave or toaster oven).

I'm going to be adding some electronics that will need to be on 24x7 and so with battery prices being less than 1/2 of when I got my 1st one (albeit for a cheap clone), I feel I can stretch my budget to accommodate the new load and provide the extra capacity.

The roof solar is now maxed out. I'm ok with the efficiency of the roof panels. (3 x 200W) They work for me because they are pretty stealthy and adding bigger panels (e.g. 3 x 300W) would make them very obvious as well as make for a very expensive upgrade.

Yes, I could have upgraded the 40A B2B to a 60A and sold the 40, but I like the idea of having an online "backup" B2B available at the push of a button. I could run just one, get 40A, and not tax my alternator at all, or run 2 and pump 80A into my battery bank in an emergency (that was the theory and I'm glad that a couple of people here confirmed that I could do that). I could have kept the 40A and added a 60A, but that would be more expensive AND I'd draw at least 100A (more likely 120A) out of my alternator, and that would have been tough on the alternator when the engine is only just idling. I would think I'd have to force the engine to fast-idle under those circumstances.

Re: old topic. I just started it yesterday! So I'm happy with the fast answers.

 

[McCarthy]

I would not pull 80 Amps out of an OEM alternator. Yours may delivery 220 peak, at high RPM, but it will overheat if your chargers try to pull around 100 amps gross at low RPM. The fan blades on the alternator will spin slowly at idle RPM and this will heat soak the insulation on the wiring, eventually causing a short.

 

[Rocketman]

If you do add the second B2B, you need to make sure the wiring can handle the load. 80 amps is a lot!

Also, I would add an on/off switch for the second one into your cockpit. That way you can disable the second one when at idle, and climbing up mountains and if you will have a long drive day (no use running the alternator hard when you have plenty of time to charge).

 

[Ample]

I would not pull 80 Amps out of an OEM alternator. Yours may delivery 220 peak, at high RPM, but it will overheat if your chargers try to pull around 100 amps gross at low RPM. The fan blades on the alternator will spin slowly at idle RPM and this will heat soak the insulation on the wiring, eventually causing a short.

I agree. I do have the optional high-output alternator though. But I'd run the 2nd one only when driving or somehow induce a fast-idle (say 1200 or 1500 rpm).

Because I travel only in Spring and Fall, there is usually plenty of sunlight and I have a good amount of solar panels.

But one time I was traveling in the late fall (thus days were a lot shorter) and it was cloudy and rainy for days. My battery bank's SoC did get down to like 30%. And so it got me thinking...

 

[Ample]

If you do add the second B2B, you need to make sure the wiring can handle the load. 80 amps is a lot!

Also, I would add an on/off switch for the second one into your cockpit. That way you can disable the second one when at idle, and climbing up mountains and if you will have a long drive day (no use running the alternator hard when you have plenty of time to charge).

I sized my coach wiring to handle well over 100A. But if you are talking about the car's wiring, yes, that's a concern. I had been contemplating what some people in the car stereo business call "The Big 3 Upgrade", which is: 1) bigger cable from the vehicle battery negative to the chassis, 2) bigger cable from the alternator positive to the battery , and 3) engine block to chassis.

I already have an on/off switch for the current B2B charger. I normally don't use it as I've got lots of solar. And yes, I'll have one for the 2nd one as well.

Re: idle. I was going to do a crude thing to get the engine to fast-idle so that RPMs will be say in the 1200 to 1500 range.

 

[Aussieguy]

I would not use main motor at fast idle as a substitute for
ground panels at camp + maybe a small generator
small generator can even be run while driving


make sure your existing B2B is producing max and wiring is sending max to battery

PS read the date incorrect when I thought was old thread

 

[OTRwSolar]

I have a Renogy 40A B2B charger and it's working well. I also have a heavy-duty alternator (220A if I remember)

Could I simply add a 2nd 40A B2B charger in parallel?
Could doing that provide 80A of charging?

I'm hoping that two B2B batteries could run simultaneously too. Thoughts?

We recently paralleled a 2nd 40a Renogy dc to dc charger with an existing 40a Renogy dc to dc charger in our truck camper. Been powering both (with some limitations) with the 220a alternator in our Ram Cummins.

Installed two DPDT (on/off/on) switches in conjunction with the D+ and LC terminals on both chargers so we could select 20a, 40a, 60a or 80a modes of operation (charge/output current).

Had some concern whether this 220a alternator could safely handle the 60a and 80a modes. We elected to cap the load on the alternator based on a maximum alternator case temperature of 220f. It's our understanding that Balmar alternators recommends using 220-225f as the maximum threshold for limiting alternator current on their alternators.

Testing revealed we could only obtain 110a of alternator output current at high idle (alternator 4000 rpm; engine 1250 rpm) while maintaining an alternator case temperature below 220f (measured via infrared gun). Ambient temps were 75f. May be able to increase alternator load above 110a while driving at freeway speeds due to better ventilation/cooling. On the flipside, we may have to decrease alternator load with high ambient temps. For what it's worth, data sheets for this alternator say it can produce 167a of current at 4000 rpm (with 200f **inlet** air temperature).

The Ram Cummins' electrical load varies from 25a (truck battery fully charged; no accessories on) to 65a (truck battery discharged, all accessories on). Based on 110a of alternator output, this leaves us with anywhere from 45a to 85a of available alternator current to power the dc to dc chargers.

With the two dc to dc chargers in 40a mode, there's a 44a load at the alternator/battery terminal. 60a mode, 72a load. 80a mode, 99a load. We're using the existing ~25 ft run of 2 gauge cable between the alternator/battery and dc to dc charger that we installed when we had one 40a unit. May upgrade this to 1/0 or 2/0 to reduce the load on the alternator--especially in 80a mode.

So, all said and done, we already knew we could safely power the dc to dc chargers in 40a mode under most conditions, even at idle (based on 2 years of use with one 40a Renogy dc to dc charger). We can safely use the 60a mode if the truck is in high idle with the truck's batteries fully charged and all accessories off. The 80a mode is only safe for about 15 min. (at high idle) before alternator case temps spike above 220f. Long-term operation in 80a mode is a no-go.

We have plans to install a 2nd OEM 220a alternator (or a single 370a aftermarket alternator) this summer. Pretty confident we can safely power both dc to dc chargers in 80a mode at idle with either upgrade.

 

[featherlite]

OTRwSolar,

Thanks for posting this very valuable, real world, data.

 

[Ample]

We recently paralleled a 2nd 40a Renogy dc to dc charger with an existing 40a Renogy dc to dc charger in our truck camper. Been powering both (with some limitations) with the 220a alternator in our Ram Cummins.

Installed two DPDT (on/off/on) switches in conjunction with the D+ and LC terminals on both chargers so we could select 20a, 40a, 60a or 80a modes of operation (charge/output current).

Had some concern whether this 220a alternator could safely handle the 60a and 80a modes. We elected to cap the load on the alternator based on a maximum alternator case temperature of 220f. It's our understanding that Balmor alternators recommends using 220-225f as the maximum threshold for limiting alternator current on their alternators.

Testing revealed we could only obtain 110a of alternator output current at high idle (alternator 4000 rpm; engine 1250 rpm) while maintaining an alternator case temperature below 220f (measured via infrared gun). Ambient temps were 75f. May be able to increase alternator load above 110a while driving at freeway speeds due to better ventilation/cooling. On the flipside, we may have to decrease alternator load with high ambient temps. For what it's worth, data sheets for this alternator say it can produce 167a of current at 4000 rpm (with 200f **inlet** air temperature).

The Ram Cummins' electrical load varies from 25a (truck battery fully charged; no accessories on) to 65a (truck battery discharged, all accessories on). Based on 110a of alternator output, this leaves us with anywhere from 45a to 85a of available alternator current to power the dc to dc chargers.

With the two dc to dc chargers in 40a mode, there's a 44a load at the alternator/battery terminal. 60a mode, 72a load. 80a mode, 99a load. We're using the existing ~25 ft run of 2 gauge cable between the alternator/battery and dc to dc charger that we installed when we had one 40a unit. May upgrade this to 1/0 or 2/0 to reduce the load on the alternator--especially in 80a mode.

So, all said and done, we already knew we could safely power the dc to dc chargers in 40a mode under most conditions, even at idle (based on 2 years of use with one 40a Renogy dc to dc charger). We can safely use the 60a mode if the truck is in high idle with the truck's batteries fully charged and all accessories off. The 80a mode is only safe for about 15 min. (at high idle) before alternator case temps spike above 220f. Long-term operation in 80a mode is a no-go.

We have plans to install a 2nd OEM 220a alternator (or a single 370a aftermarket alternator) this summer. Pretty confident we can safely power both dc to dc chargers in 80a mode at idle with either upgrade.

This is EXACTLY the real-world experience and test results I was looking for! Thanks a million @OTRwSolar !

It looks like you are also saying your Renogy 40A DC-DC charger has a 1/2 output mode (20A). I thought this was a feature only on the 60A units. Maybe I have an older version that doesn't have this.

It's disappointing to see that only 110A of alternator output is available at high idle. My vehicle is also a RAM (though it's a gas motor) and thus I may have the same alternator. I wonder how they get away with advertising it as "220A"??? Is 220A only a theoretical output? With the engine red-lined...?

By the way, do you have a good way of inducing fast-idle? I was going to use...a stick on the gas pedal...

 

[OTRwSolar]

You're welcome. Glad you found my post helpful.

All three of the RNG-DCC1212-xx-US series dc to dc chargers (20a/40a/60a) have a half-power mode that's activated by placing 12vdc on the LC+ terminal.

The 220a Mopar alternator is capable of producing 220a; however, it can only do so at 8000 rpm (engine rpm approx. 2500-2800) for a very short period of time while internal temps are very low.

We capped our alternator load at 110a to keep the alternator case temperature below 220f. The alternator is capable of producing quite a bit more current at high idle (4000 rpm), however, that additional current comes at the risk of increased heat, reduced alternator life, or possible failure. How hard you can push an alternator all boils down to your aversion to risk.

Fast idle on a Ram Cummins is activated by turning on the cruise control and making the appropriate "speed" (+/-) adjustments with the transmission in park. It's possible your Ram works the same way.