Onboard, programmable shore charger and voltage drop

[pdxfrogdog]

Hi,

Please consider the following scenario:

I'm thinking about hard mounting a programmable shore charger (Victron Blue Smart IP76 12/25A) in my vehicle to charge/maintain the batteries in the engine bay. My vehicle isn't a daily driver and I would like to protect my investment in dual Odyssey AGMs (Group 34 PC-1500). The plan is to mount the charger near an existing fuse block, behind/under the passenger front seat. I've got a 1 way run of just about 13' of dual 10AWG to the batteries. This has me thinking (or over-thinking) voltage drop. According to an online calculator, I would get a 2.2% drop for 14.7V @ 25A for my wire run, yielding 14.38V. This takes me a bit below Odyssey's recommended voltage of between14.4 and 15.0 according to their technical manual. The same calculator shows that as the amps drop, so does the voltage drop to a pretty negligible 0.08% at 1A, yielding 14.687. I realize this starts to happen pretty quickly given the battery chemistry I'm using.

My question: Should I worry about this voltage drop? Should I bump the charging parameters from 14.7 absorption to 14.8 or 14.9? Does voltage drop even matter in this context? I realize this may seem totally OCD, but I've got the charger already and it is fully programmable.

Thanks in advance!

 

[john61ct]

Use as fat wire gauge as you can stand.

Measure the voltage at the batt terminal when lots of amps are flowing and adjust your charger according to that.

Then double-check toward the end of the cycle, amps down to just a few but before the charger drops to Float, confirm still in spec.

If AHT is adjustable, tweak that until the Float transition only happens after amps have trailed down to below 0.005C

That hot environment will shorten lifespan a lot, but I guess you know that.

 

[Rando]

Your reasoning is spot on - as the current drops fairly quickly, the voltage drop will also decrease. You will still get to your target absorption voltage and once you are in absorb the current is low so the voltage drop won't matter. The end effect is that your battery will charge a little bit more slowly, but who cares when you have all the time in the world. In fact, for the shore power scenario you describe, why not use a much smaller charger or decrease the maximum charge current to just a few amps? Again, there is no need to hurry if you are parked for days.

Don't forget a fuse at the battery end of your wire run.

 

[pdxfrogdog]

Use as fat wire gauge as you can stand.

Measure the voltage at the batt terminal when lots of amps are flowing and adjust your charger according to that.

Then double-check toward the end of the cycle, amps down to just a few but before the charger drops to Float, confirm still in spec.

If AHT is adjustable, tweak that until the Float transition only happens after amps have trailed down to below 0.005C

That hot environment will shorten lifespan a lot, but I guess you know that.

Currently have 2 parallel runs of 10AWG yielding 7AWG equivalent. I did this because 1 of the runs was already installed and I'm pretty limited at the location where it's going through the firewall. I've heard of people getting 6AWG through that location with enough persistence, but as you say, this is what I can stand right now.

Not familiar with the acronym AHT?

On the hot environment issue, I'll definitely keep that in mind. The charger has 5 year warranty (Victron) and is fully rated to 40C (104F) then begins to derate 3% per 1C, with a max operating temp of 60C (140F). Fortunately I live in a mostly cool climate, but yeah the inside of a vehicle can get warm very quickly. Probably best to just pull the plug during the occasional heat wave.

 

[pdxfrogdog]

Your reasoning is spot on - as the current drops fairly quickly, the voltage drop will also decrease. You will still get to your target absorption voltage and once you are in absorb the current is low so the voltage drop won't matter. The end effect is that your battery will charge a little bit more slowly, but who cares when you have all the time in the world. In fact, for the shore power scenario you describe, why not use a much smaller charger or decrease the maximum charge current to just a few amps? Again, there is no need to hurry if you are parked for days.

Don't forget a fuse at the battery end of your wire run.

Yeah, I guess I could have saved $50 or so and some mounting space with a smaller charger. I am pretty deep into the Victron ecosystem and wanted the programmability and the feedback statistics in the Victron Connect app.

There is a 50A blue sea circuit breaker at the battery end of the wire run.

 

[john61ct]

Lifespan of the battery, engine bay is the last place you want to put an expensive one.

Absorb Hold Time

After the voltage setpoint is reached, Cc to CV transition

Amps starts to reduce as SoC finishes, 100% Full point is when current drops to a low enough flow rate, aka endAmps.

If the charger drops to Float (ending the charge cycle) prematurely ideally AHT can be extended.

Going a bit too long sometimes is better than chronic PSOC.

 

[pdxfrogdog]

Absorb Hold Time

After the voltage setpoint is reached, Cc to CV transition

Amps starts to reduce as SoC finishes, 100% Full point is when current drops to a low enough flow rate, aka endAmps.

If the charger drops to Float (ending the charge cycle) prematurely ideally AHT can be extended.

Going a bit too long sometimes is better than chronic PSOC.

Thank you for explaining.

What I'm seeing in the programming parameters are a default setting of adaptive absorb time, with a corresponding maximum time. The alternative is to turn adaptive off and have a fixed absorb time. Not sure what formula or metric Victron uses for the adaptive setting. The only thing I could find is this statement on their much more expensive inverter chargers:

The absorption time of a Phoenix Charger or Phoenix Multi will adapt itself as follows: after each period of bulk charge (= the charger has reached its maximum current) an absorption period of 20 times the the bulk charge period will follow, with a maximum set at, for example, 4 hours.

The above info is from Victron's adaptive charging white paper.

 

[john61ct]

Assume to start with, that their adaptive algorithm is fine.

Trust but verify as Ronnie said.

It likely is based off the time it takes to get to the CC/CV transition, maybe directly proportional, maybe taking into account the initial current acceptance.

Observe the amps rate falling, and confirm they go below that endAmps spec - that's your 100% Ful - before the voltage drops to Float.

If not, see if there is a "factor" to keep the algorithm but extend its hold time.

If not the go to the fallback "dumb eggtimer" mode, but then you may need to periodically adjust it as your cycling usage patterns change.

Just be aware even with lots of current, a depleted lead batt takes 6-8 hours to get back to Full.

 

[rayra]

"My vehicle isn't a daily driver"
If the use is infrequent and irregular, I'd suggest building a battery tending station in your garage and simply dismount those batteries and leave them racked until you plan to use the vehicle. As some car collectors do.
Or conversely, put the charger in the engine compartment when the vehicle is 'stored', as the maintenance device it is, and take it out when you use the vehicle.