# Current flow to batteries in parallel

#### jethro99

Joined Oct 31, 2020
6
Envision a scenario where an alternator is charging one battery. That battery is mostly charged. Small amount of current flow to it.

At a point in time a switch is closed such that a second battery is being charged by the same alternator. In parallel with the first battery. The second battery is at some state of charge less than 100%.

Is there ever any possibility of current flowing from the first to the second battery?

Apologies if this appears to be a stupid question.

#### BobTPH

Joined Jun 5, 2013
3,308
If the charging voltage does not drop from 14.6 V, then no, current will not flow out of either battery.

But that is not necessarily the case. It depends on how much current the source can supply, and how much each battery will take.

Bob

#### andrewmm

Joined Feb 25, 2011
1,464
current flow from the higher voltage to the lower voltage,
if the batteries are in parallel and one is higher voltage than the other, current will flow form one battery into the other no matter what voltage the generator is at .

#### Irving

Joined Jan 30, 2016
1,496
Not a stupid question.

Kirchhoff current laws give the answer... currents into/out of a node must sum to zero. so the 1st battery is almost charged and will be around 14v as will the output of the alternator. The current in to the node from the alternator = the current out to the battery, so sum = 0.

Now close the switch. The 2nd battery is a low SoC so its internal volts will be << 14v. therefore current will flow out of the node into the 2nd battery. and the voltage at the node will (potentially) drop depending on the output impedance of the alternator. Therefore current will flow from alternator to 2nd battery, and, if the voltage at the node has dropped below the open circuit voltage of the first battery, also from 1st to 2nd battery.

Here's a simplistic simulation that shows the 2nd battery V3 drawing ~5.2A, ~2.9A from alternator and ~2.3A from 1st battery (v2). This will continue until the voltages of both batteries have balanced and then both will charge until SoC = 100%.

Its common practice, where batteries are to be paralleled up, to connect them in parallel and allow them to equalise before connecting to a charger.

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#### jethro99

Joined Oct 31, 2020
6
I have ammeters connected such that I can see current flow from the alternator and to each battery. At no time am I seeing current flow from battery one to battery two.

Granted these are cheap hall effect ammeters. Is the situation such that the reverse current flow from the charged battery is so short in duration that it is not able to be seen on an ammeter?

If the voltage of the alternator were to drop below the voltage of battery one would current flow back to the alternator except for the diodes?

#### jethro99

Joined Oct 31, 2020
6
Practically speaking, even if current were to flow from the first battery to the second battery for a brief period of time, with current then flowing to both batteries from the alternator, would it amount to a hill of beans?

#### Irving

Joined Jan 30, 2016
1,496
I have ammeters connected such that I can see current flow from the alternator and to each battery. At no time am I seeing current flow from battery one to battery two.

Granted these are cheap hall effect ammeters. Is the situation such that the reverse current flow from the charged battery is so short in duration that it is not able to be seen on an ammeter?
It may be the ammeter doesn't register it, but I can guarantee it will under the right circumstance. If the output impedance of the alternator is very low (ie the alternator output current capability >> current charging needs of battery 2) then you might not see any flow from 1 to 2 because the alternator output voltage remains high enough. See chart below, same setup but alternator output impedance reduced from 0.5 to 0.05ohm. Now #1 remains on charge as well as #2 is charging, no flow from #1 to #2.

If the voltage of the alternator were to drop below the voltage of battery one would current flow back to the alternator except for the diodes?
Yes, exactly so.

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#### jethro99

Joined Oct 31, 2020
6
I appreciate all comments. Lots of food for thought here. Theoretical versus practical. I am trying to differentiate theoretical from the practical.

100 amp alternator. 14.6 volt set point.

Theoretically no voltage drop. Theoretically the alternator circuit voltage never goes below 14.6. Theoretically battery A is at 14.6 volts when battery B is paralleled. Theoretically the alternator is always outputting some amount of current. I presume.

With the alternator at 14.6 volts and battery A at 14.6 volts there should be no current flow. Correct?

When the switch is closed in order to parallel battery B with battery A, current should flow from the alternator only to Battery B. Given that the alternator and battery A are at the same voltage. Capiche?

Now, there is some voltage drop that I can see. As noted however, I am not seeing any current flow from battery A. Presumption being that it happens so quickly and for such a short period of time that it not measurable.

Assuming that current does flow from battery A and is for a short period of time, does it make any practical difference in the real world?

Here is the situation. Marine application. Start battery and house battery. Normally electrically isolated from each other. A voltage sensitive relay is used to parallel the two batteries but only when the alternator is providing voltage and current such that the relay sense circuit detects 13.6 or more volts. Relay closes and puts the two batteries in parallel such that both are charged.

#### andrewmm

Joined Feb 25, 2011
1,464
First a question, can your amp meters measure current in both directions ?

If you want to test practical against theory,
then lets take things to a limit where the answer will be evident.

For first test, Ignore the generator, take it out of circuit,
connect a fully charged battery to a discharged battery,
( stand back , in case it explodes )

You will see that current flows from the higher voltage to the lower voltage.

Now if your system does not show this , then I would say its the test that has to change,

Try it and come back to us , but please be carful, it can explode,

#### nsaspook

Joined Aug 27, 2009
8,382
"Try it and come back to us , but please be careful, it can explode"

#### jethro99

Joined Oct 31, 2020
6
Yes. The amp meters have an indicator to denote current in an opposite direction.

I have paralleled a fully charged battery in the past with a less than fully charged battery. Current obviously flowed from the battery with the higher voltage to the battery with the lesser voltage. Can't recall one being completely discharged but it was likely to have been.

The more that I think about it, I believe that I have jumpered a fully charged battery in one automobile (with the alternator of the automobile running) to another automobile that had a dead battery that would not crank the motor. On several occasions over my too many years on this earth. Still have my eyes and such. Have read/heard of bad stuff happening but never encountered it personally.

It is the alternator being in the loop, keeping the voltage above both batteries, that has me questioning all of this. This being current flow from battery A over to battery B but possibly against the current flow from the alternator.

#### Irving

Joined Jan 30, 2016
1,496
If the alternator can output 14.6v off load and still stay above say 13.6v on 100A load then its output impedance is 1/100ohm = 0.01ohm. If Vbatt1 is currently near to 14.6v so current into it is minimal and you attach battery 2 thats close to 0% SoC so its internal volts is say 11v with an internal impedance of 0.1ohm (typical for a discharged battery) then the current into battery 2 will be 32A and the alternator output volts will drop to 14.3v, a mere 0.3v. The current drawn from battery1, if any, will be negligible, and probably won't show on those meters.

While, as I said previously, there are circumstanes where there will be current drawn from battery 1 its generally going to be small and time limited. My thoughts are, why are you worrying about it, its mainly a non-issue.

#### jethro99

Joined Oct 31, 2020
6

You are absolutely correct. Operationally, it is a non-issue.

The question only came up because a pedant on a boating forum was trying to make a mountain out of a mole hill. Trying to make up an argument for the sake of an argument. The innerweb is full of those characters.

I came over here hoping to have a rational conversation. Which has been the case.

#### LowQCab

Joined Nov 6, 2012
510
There is a simple way to state what will happen.

A fully charged Lead-Acid Battery can not physically put out more than about ~13.8V.

If that Battery is connected to a Power Source that is Regulated at 14.6V,
then ALL Current flow will ALWAYS be from the Power-Source to the Battery.
( from the higher-Voltage to the lower-Voltage ).

As long as the Alternator Output Voltage remains at 14.6V,
you can connect as many Batteries in parallel as you want,
and ALL of them will be charged at the same time,
and ZERO Current will flow from Battery to Battery.

As soon as the Alternator Voltage Drops below ~13.8V,
the Current flow between the Batteries "MAY" change,
and the charge-level-percentage will "balance-out" between the Batteries over time.

If the Batteries are not physically "Identical" for some reason,
and the Alternator stops working,
eventually, over time, the energy available will be dictated by the weakest Battery.
This is why it is not "best-practices" to permanently parallel Lead-Acid-Batteries,
even though it is common practice in Diesel-Trucks today.
.
.
.

#### BobTPH

Joined Jun 5, 2013
3,308
If the Batteries are not physically "Identical" for some reason,
and the Alternator stops working,
eventually, over time, the energy available will be dictated by the weakest Battery.
That would be the case if the batteries were in series, not in parallel.

But b

#### nsaspook

Joined Aug 27, 2009
8,382
That would be the case if the batteries were in series, not in parallel.

But b
It happens to some extent in parallel connections too with LA blocks (series) of cells batteries (like a typical 12dc SLA battery) in high cycle applications like daily solar. Other applications like Standby Emergency Power service (with batteries designed for that type of service in float) parallel connections are usually not sensitive to long term capacity variations.

To keep the string balanced a few things are needed like:
Making sure that the current is shared equally between the batteries so that the they are all put under equal stress.
Making sure that each of the batteries has enough "absorb time" so that all the batteries will be fully charged but not overcharged.

The weak battery (one bad series cell in the battery) becomes a net energy sink from the other good cells that can eventually cause large SG differences between cells in parallel battery banks with increasing variations in SoC and capacity reductions between cells during repeated recharge/discharge cycles.

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#### BobaMosfet

Joined Jul 1, 2009
1,768
I appreciate all comments. Lots of food for thought here. Theoretical versus practical. I am trying to differentiate theoretical from the practical.

100 amp alternator. 14.6 volt set point.

Theoretically no voltage drop. Theoretically the alternator circuit voltage never goes below 14.6. Theoretically battery A is at 14.6 volts when battery B is paralleled. Theoretically the alternator is always outputting some amount of current. I presume.

With the alternator at 14.6 volts and battery A at 14.6 volts there should be no current flow. Correct?

When the switch is closed in order to parallel battery B with battery A, current should flow from the alternator only to Battery B. Given that the alternator and battery A are at the same voltage. Capiche?

Now, there is some voltage drop that I can see. As noted however, I am not seeing any current flow from battery A. Presumption being that it happens so quickly and for such a short period of time that it not measurable.

Assuming that current does flow from battery A and is for a short period of time, does it make any practical difference in the real world?

Here is the situation. Marine application. Start battery and house battery. Normally electrically isolated from each other. A voltage sensitive relay is used to parallel the two batteries but only when the alternator is providing voltage and current such that the relay sense circuit detects 13.6 or more volts. Relay closes and puts the two batteries in parallel such that both are charged.
You're not thinking about it correctly. Voltage is a potential. If you have 2 batteries in parallel, and you have a charger that is outputting more voltage than either one of them, then if both batteries are in parallel, they are both being charged. The only way battery one could overcome the charger and thus charge battery two, is if battery one had a higher voltage than the charger.

#### LowQCab

Joined Nov 6, 2012
510
The best way to set-up Twin-Batteries in a Car or Boat is to .........

1) Purchase an Alternator with Dual-Bridge-Rectifiers, ( available from "MechMan" Alternators ),
or
2) Tap into the 3-Field-Windings on your existing Alternator,
( Using 3- 10-gauge Wires, works just fine up to around ~100-Amps, don't ask me why ),
and add on a second, External, 3-Phase Bridge Rectifier, for the Second Battery System.

This will keep the 2-systems completely separated without incurring the losses of a Battery-Isolator,
and, requires no switching of any kind.
This is the set-up I have on my Truck,
along with 2-Optima SLA Batteries, and it works beautifully so far for ~4-years.

Actually I use 4-additional 3-Phase-Bridges, mounted together on a large Heat-Sink.
3- of them are set up to supply Heavy-Loads, like the 4-Radiator-Fans, and the AC-Blower,
and everything else that should not be powered when the Engine is not running.
This eliminates having to Switch High-Current-Loads,
Power is available as soon as the Engine starts.
This also spreads-out high-Current Wiring, instead of concentrating it all in one spot,
and increases reliability.

So, if I do something stupid, and run my Main-Battery dead,
I can easily give myself a Jump-Start, and be on my way.