Parallel DC chargers not working

[Jacob Scoville]

Hello! I’m looking to charge my 400 AH battery bank with a second charger while driving. Currently I use a 50 amp renogy DC to DC charger/MPPT. I’ve upgraded from a 150 to 270 amp alternator to increase the alternator charging (I have high load stuff like 3D printer, pressure cooker, tons of batteries for work) and I connected a 60 amp DC charger but it doesn’t seem to want to work in parallel. I’m wondering if I need to add a diode or something somewhere in the system. I have a cable from the positive on the starter to a bus bar (with a fuse between) , and the right size CB’s between the bus bar and each charger. All negatives go to the same negative bus bar. I’m wondering if I need diodes between the positive on each charger and the battery to prevent some kind of backfeed. When charger B is turned on I notice the input amps on the renogy reduce while the output stays at 50 amps which leads me to believe the renogy is somehow sensing charger AB’s output amps and thinking they are it’s own. Is that because neither charger has isolated negatives?

 

[Will Prowse]

Can you post pictures of the components and how they are wired? And pictures of the alternator connections and voltages present on the conductors supplying the charger?

And the settings of both chargers

 

[mikefitz]

You DC to DC chargers won't be affected by a common negative.
I suspect you have voltage drops in the input path to the DC chargers . With both chargers functioning the draw fron the alternator will be in the region of 130 amps, perhaps higher. When you consider vehicle loads even a 270 amp alternator at low engine speed may be heavily loaded.
Measure the volts along the path between alternator and DC charger input. Volt drops across CBs. Is not unusual
Up grading an alternator may need up graded cables and frame connections.

 

[Jacob Scoville]

You DC to DC chargers won't be affected by a common negative.
I suspect you have voltage drops in the input path to the DC chargers . With both chargers functioning the draw fron the alternator will be in the region of 130 amps, perhaps higher. When you consider vehicle loads even a 270 amp alternator at low engine speed may be heavily loaded.
Measure the volts along the path between alternator and DC charger input. Volt drops across CBs. Is not unusual
Up grading an alternator may need up graded cables and frame connections.

Just to clarify, in tests I was only using a few amps on charger number 2 so total amps was not an issue.

When I put in the new alternator a few months ago I upgraded the OEM 2 and 4 AWG cable to 4/0 cable everywhere in the engine bay (positive to starter and all ground points). Alternator is rated for 200 amp output at idle RPM’s which is 50 amps more than the OEM was at highway RPM’s and the OEM handled the renogy fine for years. The inputs to each charger are coming from the same positive and negative bus bars inside so it’s definitely not anything to do with wiring not supporting the amps.

 

[Vigo]

The only way the Renogy input amps could reduce while output remained at 50amps is for input voltage to go up or output voltage to go down. One of those things has to be true, otherwise this is just a measurement/reporting error. Please don't take this as an insult but if charger B was 'turned down' and did not have reverse polarity protection, having the output polarity wrong could cause that symptom. But most chargers would let you know if polarity were wrong unless it were something like the 'dumb' meanwell knockoff power supplies i screw around with.

Your wiring sounds pretty darn good. I upgraded the alt in my RV from ~130 to 220a and with original positive charge wire it did about 70amps at idle. I paralleled a second conductor onto it and charging amps at idle went to 130a. Didn't touch the ground but the ground conductor was already much larger because it's shared with the starter circuit. Im happy with the 130a but my wiring upgrade is a joke compared to 4/0 ?

 

[Jacob Scoville]

The only way the Renogy input amps could reduce while output remained at 50amps is for input voltage to go up or output voltage to go down. One of those things has to be true, otherwise this is just a measurement/reporting error. Please don't take this as an insult but if charger B was 'turned down' and did not have reverse polarity protection, having the output polarity wrong could cause that symptom. But most chargers would let you know if polarity were wrong unless it were something like the 'dumb' meanwell knockoff power supplies i screw around with.

ISDT P30

High power charger offers dual channel capabilities - charge at up to 30A per channel!

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I’m guessing I screwed up and used a charger meant for an isolated setup with a non-isolated setup (the renogy).

Polarity is fine or else I wouldn’t be able to operate the second charger from one of the sets of wires.

A bummer because the renogy is nice and compact for offering 50 amps from my alternator when I want it and MPPT capabilities and lasted for nearly 5 years and victron seems to be lagging behind with amp output (sterling started to offer a variety of high output solutions in the last year). I don’t really have space or funds for 3 30 amp victron chargers and a MPPT.

As a temporary solution I’m thinking about charging my two 206 amp batteries separately and using one for the inverter and one for everything else , that way I could have an isolated setup for the second charger and a non-isolated setup for the renogy. None of my devices use anywhere near the max amp output of one battery.

 

[Vigo]

Well, im doing a relatively silly thing of having my RV inverter switch between the lead-acid 'starter' battery system (~270ah worth of 3 mixed FLA in parallel, lol) or my 480ah of lifepo4, and when i desire to 'alternator' charge the lifepo4 im using the output of the 120vac inverter when powered by the FLA, through a rectifier/filter section, into an mppt, to charge the lithiums. So take solace that sometimes failing (on first try) to accomplish something the right way might still be more appealing than successfully accomplishing it the wrong way. Your impending success on round two is going to look a lot better than my success!