Ohmmu Battery Update and Inverter Question

@ChapmanF What are the potential hazards if I simply connect the cables to the battery and the Prius? The car will be smart enough not to overcharge it right? I can test the volts and amperage in the real world.

I’m looking at the @black_jmyntrn diagram below. Without adding a load wouldn’t the batteries just charge until the Prius detected they were “full”?

 

That rig looks good... Isolator is key! Not sure of the details of its functions. As for Prius knowing battery is fully charged, I've not read of that capability. I think because lead acid batteries can handle a constant charge even when full they don't need to have a feature to turn off the charge going to the 12v if it is at full charge. I could be wrong, but I doubt it.

 

The car has absolutely no such smarts. It will put 13.6 to 14.8 volts onto the 12 volt bus, and your battery will do whatever it is going to do when exposed to that voltage. There was an earlier post in this thread with some speculative info about when the car adjusts the voltage within that range.

 

Seems like getting the amount of amps sent to the 12v on a really long road trip with a fully charged 12v would confirm that there's nothing smart about the charging signal for the 12v. But when it comes to a smart charger its the change in amps not the change in voltage.

I do know that over at Insight Central the original Honda Insights had a smarter 12v charge signal coming from the hybrid battery so it wasn't always on and lots of people who work on those cars would snip a specific wire to make the 12v charge constant because they decided the smart charge function was too dumb to trust.

 

Still learning. Here is the 60 amp version of what we see in the video. Do we know the Prime can consistently output 60 amps (if the rest of the car is under minimal loads)? 12V 60A DC to DC On-Board Battery Charger | Renogy Solar

Reference to the Prius producing 100 amps maximum in the video further below.

Of course @black_jmyntrn isn’t using this kind of equipment,

 

If you're pulling a boulder from here to Omaha and you need it to get there sooner, you need the boulder to move faster. But your only control over how fast it moves (read: amps) is how hard you pull (read: volts).

The battery is going to have a certain curve for how many amps will flow given a certain applied voltage and a certain state of charge. A smart charger may be targeting a certain number of amps, but its only tool for hitting that target is to raise or lower its output voltage until the battery is conducting that many amps.

 

Thanks. This video an device seems helpful too and adds a potential solar capacity. Comments welcome.

Schematic: RV Solar Power Blue Prints - Mobile Solar Power Made Easy!

 

... but also look at RGeB's datalogs from a RAV4 hybrid. That model has the battery state sensor assembly, like a Gen 4 Prius, so it is not a perfect fit in this Gen 3 thread, but at least in that model there seems to be an interesting strategy for peeking at the battery state of charge during driving. I have never seen an equally detailed datalog for Gen 3 Prius (or Gen 2 or 1) to see whether a similar strategy is in use there.

 

So you're saying that when a charger is sending a charge to a battery that's full and the amps are at a trickle it's less than the ~14.7v signal it's sending and when the battery is getting an 8 amp charge and a long way from being full its getting more than the ~14.7v signal it says its sending? Why wouldn't the voltage meter on charger show the difference in voltage?

 

To a first approximation, you can think of a battery as a voltage source equal to its open-circuit voltage, in series with a certain amount of internal resistance. The open-circuit voltage tends to rise as the battery goes from discharged to charged.

To charge the battery, you put a voltage across it. The battery's own open-circuit voltage pushes back. Your voltage has to be higher, and because of that voltage difference a current will flow in the charging direction. In this approximate model, that voltage difference—how much your charge voltage exceeds the battery's opposing voltage—is determining the flow of current through that equivalent internal resistance, much as you would think from Ohm's law.

As the battery goes toward full charge, its open-circuit voltage rises. So even if you are charging at a constant voltage, the difference between your charge voltage and the opposing voltage decreases, and that smaller voltage difference drives a smaller current through the battery. Eventually, if the battery's open-circuit voltage rises very close to your charge voltage, the voltage difference will be very small, and so will the current.

So that's what happens if you use a constant-voltage charger. A large current will flow when you first connect a drained battery, and the current will taper off as the battery charges. The charger isn't responsible for that; it's just producing its constant output voltage, and what happens, happens.

Now if you use a charger that can be set to a target charge rate in amps, that charger will measure the current flowing, and if it is above the target, the charger will reduce its voltage until the target current is flowing. If the current goes below the target (as expected as the battery charges), the charger can increase its output voltage to keep the current on target.

'Smart' chargers can often operate in a constant-voltage and a constant-current mode, and switch between them in different phases of charging.

 

@ChapmanF, @Elektroingenieur, or anyone else. Can you help me think about what happens if I buy this: DCC50S DC-DC MPPT On-Board Battery Charger | Renogy Solar. I posted the video above about what this device does.

If I connect it with large gauge cables between, on one end the 12v of my Prius Prime parked in “Ready” mode while turning off all possible additional loads (AC, lights, etc.), and on the other a 12v, 100 amp, 70 amp hour, AGM battery at 50% charge.

Can the Prime’s DC to DC converter handle a potential sustained 50 amp load? Does the vehicle need to be moving to cool the Prime’s converter? Or would this successfully charge the battery with little to worry about? (Once charged I would remove the extra battery. Not intended to be permanent.)

 

Update: This person seems to have done this: https://diysolarforum.com/threads/hybrid-installation-of-renogy-dcdc-with-mppt.26678

Thoughts?