Renogy 20A dc to DC report

theoldwizard1 wrote:
If you are towing or using a truck camper, the B+ wire on the 7 pin trailer harness is almost always wired to a KEY-ON circuit. Now additional wiring required.

Yes, B+ on the 7-pin is a possible option. Several downsides to using a simple ignition on approach, however. The dc to dc charger would still continue to operate if the ignition is left on without the engine running--potentially depleting the battery. Plus, it would also continue to operate if the alternator fails. Having a very high amperage drain on your battery if your alternator fails, in the middle of nowhere, would not be fun.

We much prefer using a Battery Isolator with a 13.3v close/on, and 12.8v release/off relay. The BI prevents the dc to dc charger from operating unless the alternator is operating. If you turn on the ignition without the engine (alternator) running you're protected. If the alternator fails while you're driving, you're also protected.

The BI can be installed pretty much anywhere. Next to the dc to dc charger or on the TV. One option, wire the BI inline with the power cable feeding the dc to dc charger. Another option, mount the BI next to the charger and parallel off the input terminal of the dc to dc charger with a small wire and use the BI to simply open/close voltage to the Renogy's ignition trigger wire. We prefer the latter. One less connection/terminal on the primary power cable is a good thing.

otrfun wrote:
The Renogy ignition trigger lead and/or commonly used battery isolator accomplishes would you're describing. If the Renogy ignition trigger lead detects a "nominal" 12vdc, the charger turns on. If the 12vdc goes away, it turns the charger off. A battery isolator operates in a similar fashion, but it's much more voltage selective.

If you are towing or using a truck camper, the B+ wire on the 7 pin trailer harness is almost always wired to a KEY-ON circuit. Now additional wiring required.

otrfun wrote:
What pianotuna is describing is a 3-4a drop in output charge current at idle (vs. driving).

I mis-understood! I would be unhappy as well. I don't think that is in the spec sheet !

pianotuna wrote:
otrfun,



In the RV the wire length is about 12 feet due to the dc to DC having to be located under the passenger's seat. It is #8 wire which is more than adequate for 30 amps input (about 6 feet on input side).

The renogy does have a cooling fan.

hold on here, #8 gage is the bare minimum size for a 2% voltage drop at a max lenght of 6 feet, I would hardly call that more that adequate, thats pushing the limit.

I do remember you saying it was to hard to upgrade the wire so if your happy with it thats fine but Otrfun is right, a larger cable would reduce the voltage drop and lower your inlet current.

otrfun,

Back when I first had a 2500 watt inverter, I'd often run the water heater on electric. I monitored voltage when doing so and when it reached 12.2 I'd shut down and allow the battery bank to recover. I found I could sustain a 1:3 where I'd run the heater for 20 minutes and then leave it off for 40 minutes.

I'm still on the OEM alternator.

theoldwizard1 wrote:

otrfun wrote:
The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop in input current to the dc to dc charger when the engine is idling at, say, 750rpm vs. driving at 2000 rpm.

It is not an amperage issues.

As long as the alternator can sustain the MINIMUM input voltage to the DC-DC charger, it is a non-issue. Most of these chargers have a way to determine if the engine is running or not to prevent discharging the vehicle starting battery OR they need to be wired to a KEY-ON circuit.

An adjustable minimum input voltage might be useful.

The Renogy ignition trigger lead and/or commonly used battery isolator accomplishes would you're describing. If the Renogy ignition trigger lead detects a "nominal" 12vdc, the charger turns on. If the 12vdc goes away, it turns the charger off. A battery isolator operates in a similar fashion, but it's much more voltage selective. It closes its relay (providing power to the Renogy dc to dc charger) when voltage reaches approx. 13.3v (alternator active) and then opens it (releases) when voltage drops to approx. 12.8v (alternator offline). These are simple on/off devices.

What pianotuna is describing is a 3-4a drop in output charge current at idle (vs. driving). Neither the Renogy 12vdc ignition trigger lead, nor a battery isolator would cause this. They simply turn the Renogy dc to dc charger on or off. Nothing more, nothing less. They do not interact with the Renogy's internal voltage/current regulation circuitry in any way.

otrfun wrote:
The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop in input current to the dc to dc charger when the engine is idling at, say, 750rpm vs. driving at 2000 rpm.

It is not an amperage issues.

As long as the alternator can sustain the MINIMUM input voltage to the DC-DC charger, it is a non-issue. Most of these chargers have a way to determine if the engine is running or not to prevent discharging the vehicle starting battery OR they need to be wired to a KEY-ON circuit.

An adjustable minimum input voltage might be useful.

pianotuna wrote:
otrfun,

Out put voltage is set for 14.7 and input is about 13.2.

Which dc to DC unit are you using?

40a Renogy.

pianotuna wrote:
. . . the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

pianotuna wrote:
. . . I don't have an easy way to measure input amps to the unit . . .

I'm confused. Explanation?

Current (along with voltage) measurements at the alternator/battery (power source for the dc to dc charger) and the input terminals of the dc to dc charger are, for us, 2 of the 4 critical data points necessary to determine the overall quality and efficiency of any given dc to dc charger installation. The other 2 are the output terminals and battery terminals on the charge side.

pianotuna wrote:
. . . I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit . . .

The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop/rise in output charge current when the engine is idling at, say, 750rpm vs. 2000 rpm.

otrfun,

Out put voltage is set for 14.7 and input is about 13.2.

Which dc to DC unit are you using?

pianotuna wrote:
otrfun,

The bench testing used #4 wire input and the length was inches.

30 amps in and 20 out is not a measurement of efficiency--because we are dealing with mppt boosting the voltage, i.e. swapping amps for volts.

I've set the unit to 14.7 volts output to maximize charging.

In the RV the wire length is about 12 feet due to the dc to DC having to be located under the passenger's seat. It is #8 wire which is more than adequate for 30 amps input (about 6 feet on input side).

I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit.

I'm more than satisfied with these results and I'd recommend dc to DC charging.

On a disappointing note, Victron, who make excellent equipment, in general, chose to not have a cooling fan on their dc to DC charger. Some reports say it gets too hot to touch after operating for an hour.

The renogy does have a cooling fan.

Yes, input and output voltage delta of a typical dc to dc charger is a factor. However, the delta is not enough to substantially change the final calculation based on current alone. For general discussion it's more than accurate.

The typical dc to dc charger install will have less than a 10% voltage delta. If it's a quality install, it will be significantly lower.

In our case, our 40a dc to dc charger had an input voltage from the alternator of 14.2v with the output charge voltage set to 14.4v (voltage increased from 13.6v to 14.4v during the charge process). Max input current was 44a, max output current 40a. ~90% efficiency even with the voltage delta taken into consideration.