Battery charging from shore power?

BFL13 wrote:

You do need to have the charger's voltage higher than the battery's or no current will flow at all, but that is not the issue.

Yes it IS. That is exactly the issue, expanded a tiny bit.

BFL13 wrote:
"You don't need to be unsure here at all. If the supply can't provide the current that the load wants, the voltage at the load goes down.

You are trying to make it too difficult: A higher voltage applied to a battery will charge it more. If the charger cannot supply the necessary current to do that, the voltage will not BE higher"

Now that I do not agree with at all! The battery voltage will rise as it is being recharged even if the recharging current is less than the battery's "acceptance rate" at the time.

You can recharge with a 20 amper even if the battery would accept a 40 amper.

If it is a 20 amp charger doing 20 amps, making it a higher voltage 20 amper won't make it do more amps. However, if the charger is only doing 15 amps at 14v, then raising its voltage to 14.8v could make the amps go up to its 20 amp limit (as long as the battery acceptance rate for that SOC and 14.8v is 20 amps or more)

You do need to have the charger's voltage higher than the battery's or no current will flow at all, but that is not the issue.

Vulcan Rider (Edit: this was an incorrect attribution prevously) has it correct. The "higher voltage 20A charger" will not actually be producing a higher voltage at the battery (given the same SOC, etc.) if both are limited by current.

Current and voltage are not independent of each other at the battery. If you apply a certain charging voltage (again, assuming a specific SOC, temperature, etc.), then a specific current will flow. If you have a certain current, the voltage will be a specific value.

Similarly, with a specific hose nozzle, if you supply water at a specific pressure (at the nozzle), you'll get a certain water flow rate. If you have a specific flow rate, the pressure at the nozzle will be that value. You can't independently set the pressure and the flow rate--they depend directly on each other for a specific nozzle setting. Changing the nozzle setting will, of course, vary this relationship, just as the relationship between electric current and voltage will change as the battery charges.

"You don't need to be unsure here at all. If the supply can't provide the current that the load wants, the voltage at the load goes down.

You are trying to make it too difficult: A higher voltage applied to a battery will charge it more. If the charger cannot supply the necessary current to do that, the voltage will not BE higher"

Now that I do not agree with at all! The battery voltage will rise as it is being recharged even if the recharging current is less than the battery's "acceptance rate" at the time.

You can recharge with a 20 amper even if the battery would accept a 40 amper.

If it is a 20 amp charger doing 20 amps, making it a higher voltage 20 amper won't make it do more amps. However, if the charger is only doing 15 amps at 14v, then raising its voltage to 14.8v could make the amps go up to its 20 amp limit (as long as the battery acceptance rate for that SOC and 14.8v is 20 amps or more)

You do need to have the charger's voltage higher than the battery's or no current will flow at all, but that is not the issue.

BFL13 wrote:

Vulcan Rider wrote:

BFL13 wrote:

You can guess at that, but you really need to see the amps to the battery for each method over some time.

It is not a guess.

A higher voltage measured AT THE BATTERY will charge the battery more/faster.

You also have to consider the amps the power source can supply, but given that those are equal, I am unsure who is right here.

You don't need to be unsure here at all. If the supply can't provide the current that the load wants, the voltage at the load goes down.

You are trying to make it too difficult: A higher voltage applied to a battery will charge it more. If the charger cannot supply the necessary current to do that, the voltage will not BE higher.

BFL13 wrote:

Vulcan Rider wrote:

BFL13 wrote:

You can guess at that, but you really need to see the amps to the battery for each method over some time.

It is not a guess.

A higher voltage measured AT THE BATTERY will charge the battery more/faster.

You also have to consider the amps the power source can supply, but given that those are equal, I am unsure who is right here.

With my 7-pin , using the Trimetric in the trailer to read the amps, I get about 10 amps at start up, (truck at 14.5v) which tapers to 6 amps within four minutes.(truck at 14v) I did not take trailer battery voltages then however, so I will do that to confirm.

OTOH, my 13.8v 7355 converter does about 35 amps at first at the batteries, then quickly tapers into the 25 amp range for the longer haul doing a recharge (it did about half those amp with the original wiring, but I did an up grade on that)

So to settle this, I need to run the 7-pin and now take the trailer battery voltages along with the amps.

Ok, trailer battery before start 12.8 volts, connect 7-pin, start truck, back to trailer and the Trimetric. Shows 31.1 volts 7 amps then 13.4 v 5 amps, etc. Out at truck, engine batt posts read 13.94 volts.

So as the trailer batt voltage climbs the rest of the way to what the truck is, less voltage drop, the amps will be tapered right down as the voltage spread shrinks.

Anyway, I cannot test for what amps you would get with trailer batts at 14.3 as Vulcan Rider does with my set-up, to compare with what the 13.8v converter does.

Vulcan Rider wrote:

BFL13 wrote:

You can guess at that, but you really need to see the amps to the battery for each method over some time.

It is not a guess.

A higher voltage measured AT THE BATTERY will charge the battery more/faster.

You also have to consider the amps the power source can supply, but given that those are equal, I am unsure who is right here.

With my 7-pin , using the Trimetric in the trailer to read the amps, I get about 10 amps at start up, (truck at 14.5v) which tapers to 6 amps within four minutes.(truck at 14v) I did not take trailer battery voltages then however, so I will do that to confirm.

OTOH, my 13.8v 7355 converter does about 35 amps at first at the batteries, then quickly tapers into the 25 amp range for the longer haul doing a recharge (it did about half those amp with the original wiring, but I did an up grade on that)

So to settle this, I need to run the 7-pin and now take the trailer battery voltages along with the amps.

When I upgraded from my Parallax/Magnatek 7345 to a Progressive Dynamics 4655, I also upgraded my wires from 6g to 4g.

Six AWG wire. 600 dollar HUBBELL 200 ampere socket and plug. 42' round trip alternator to batteries. 14.1 volts showing at alternator terminal. 13.1 volts showing on 800 A/H bank. Seventeen amperes measured. I wish it would have been a quicker charge. I should have wired with 2 AWG.

BFL13 wrote:

You can guess at that, but you really need to see the amps to the battery for each method over some time.

It is not a guess.

A higher voltage measured AT THE BATTERY will charge the battery more/faster.

Vulcan Rider wrote:

MEXICOWANDERER wrote:
The trailer batteries charge 10 times SLOWER when the truck is recharging them.

Maybe true if some configurations but not mine.

When the truck is running, the "trailer" batteries show about 14.3 volts. When the gen or shore power is active, they show in the mid 13's. I'd say the truck is charging them more.

You can guess at that, but you really need to see the amps to the battery for each method over some time.

The 7-pin can be a bit higher in voltage at first, but be really lame in amps due to long thin wire, and also by the action of the alternator reacting to heating and to the truck battery SOC rising quickly, so the alternator drops to 13.8 quickly too, reducing amps to the batt via 7-pin to a trickle.