# lead acid battery charging problem. current drops causing charge to slow down

#### sharkai

Joined Jul 29, 2016
5
Hi guys! I am new to the forum.
Basically I have an uncle who is doing a little solar project.

He has a problem where he says that when he is the charging his batteries with a controller, the voltage near the terminals apparently increases before the batteries reach that potential (the batteries are charging but its a fake potential across the electrodes or something). The charge apparently takes some time to spread across the batteries.
This causes the controller to use lower current to charge the batteries. Which slows down the charging and he can't completely charge the batteries in that time.

I don't know anything about batteries, so I don't know what's going on here. Google hasn't helped much.
What is going on here? How does a controller detects batteries potential anyway?
Is fake potential across battery terminals actually a thing. Does charge takes time to spread across the battery?
And how can this problem be solved? So that the current doesn't drop at least till batteries are 70% charged.

#### jpanhalt

Joined Jan 18, 2008
11,088
Hi guys! I am new to the forum.
Basically I have an uncle who is doing a little solar project.

He has a problem where he says that when he is the charging his batteries with a controller, the voltage near the terminals apparently increases before the batteries reach that potential (the batteries are charging but its a fake potential across the electrodes or something). The charge apparently takes some time to spread across the batteries.
This causes the controller to use lower current to charge the batteries. Which slows down the charging and he can't completely charge the batteries in that time.
When charging, the voltage applied to a battery has to be higher than the voltage provided by the battery; otherwise, no electrons flow into the battery.

I don't know anything about batteries, so I don't know what's going on here. Google hasn't helped much.

John

#### Alec_t

Joined Sep 17, 2013
12,000

#### sharkai

Joined Jul 29, 2016
5
When charging, the voltage applied to a battery has to be higher than the voltage provided by the battery; otherwise, no electrons flow into the battery.

John
The batteries are charging.
The situation is basically this
Power supply voltage 14V
Battery voltage 7V
But when the batteries are charging. The controller sees higher voltage than 7V (the fake potential I was talking about) thus lower the current because I think it enters topping charge state. This causes the slow the charging issue, which doesn't allow the battery to fully charge in the required time.

Thanks for the link. I am going through that now.

#### nsaspook

Joined Aug 27, 2009
8,263
Hi guys! I am new to the forum.
Basically I have an uncle who is doing a little solar project.

He has a problem where he says that when he is the charging his batteries with a controller, the voltage near the terminals apparently increases before the batteries reach that potential (the batteries are charging but its a fake potential across the electrodes or something). The charge apparently takes some time to spread across the batteries.
This causes the controller to use lower current to charge the batteries. Which slows down the charging and he can't completely charge the batteries in that time.
You don't have adequate information for a good answer but one thing that might be a problem with good batteries is the charging cables voltage drop. The more expensive solar chargers have a remote sensing line to directly monitor the battery voltage at the terminal instead of the charger output terminals to compensate for voltage drop. If a too small (and/or too long) wire is used so the voltage drop between the charger and battery at the desired charging current is in the hundred mill-volt range or above the charge time to a full charge will be increased until the cell internal voltage potential rises, reduces the charge current flow and wiring voltage difference drops. If you reduce this voltage drop to near zero by reducing the total cable resistance the cells will be able to receive full current at the charge controller set-point.

#### nsaspook

Joined Aug 27, 2009
8,263
The batteries are charging.
The situation is basically this
Power supply voltage 14V
Battery voltage 7V
But when the batteries are charging. The controller sees higher voltage than 7V (the fake potential I was talking about) thus lower the current because I think it enters topping charge state. This causes the slow the charging issue, which doesn't allow the battery to fully charge in the required time.

Thanks for the link. I am going through that now.
If these are 12V batteries at 7V they are dead and gone with probable high-resistance.

#### sharkai

Joined Jul 29, 2016
5
O yeah I forgot to mention
Normally it is recommended the charging current to be around 2 to 4 amperes or something
We are using much higher current like near 30 amperes which is probably why the jump from constant current charge to topping charge state.
Time for a trip to the University.
Been there
Is there way to maintain constant current charge state for longer period?
That's what I want.
You don't have adequate information for a good answer but one thing that might be a problem with good batteries is the charging cables voltage drop. The more expensive solar chargers have a remote sensing line to directly monitor the battery voltage at the terminal instead of the charger output terminals to compensate for voltage drop. If a too small (and/or too long) wire is used so the voltage drop between the charger and battery at the desired charging current is in the hundred mill-volt range or above the charge time to a full charge will be increased until the cell internal voltage potential rises, reduces the charge current flow and wiring voltage difference drops. If you reduce this voltage drop to near zero by reducing the total cable resistance the cells will be able to receive full current at the charge controller set-point.

#### sharkai

Joined Jul 29, 2016
5
If these are 12V batteries at 7V they are dead and gone with probable high-resistance.
Lol.
That was just an example
I don't know exact value right now.
I will come back with exact value soon.

#### jpanhalt

Joined Jan 18, 2008
11,088
Your example was awfully detailed for not being a real battery. Since you mentioned solar, I assumed (perhaps falsely) that they were lead acid batteries.

Can we move on to a real situation?
1) What battery chemistry are you pondering?
2) How many cells of that chemistry?
3) What is the usable watt capacity of the batteries?
4) What is the maximum charge rate?

John

#### Ylli

Joined Nov 13, 2015
1,037
O yeah I forgot to mention
Normally it is recommended the charging current to be around 2 to 4 amperes or something
We are using much higher current like near 30 amperes which is probably why the jump from constant current charge to topping charge state.
No kidding. A battery has an internal resistance - and a discharged battery can have a significant internal resistance. When you input a charging current to that battery, a voltage is developed across the internal resistance, and the batteries terminal voltage will rise without any actual charging having occurred. When you charge at such and excessive rate, the large drop across the internal resistance will look to the charger like the battery is fully charge and the charger may very well drop to a maintenance rate.

Example with an battery internal resistance of 0.1 ohms, a 30 amp charge will cause the terminal voltage to rise 3 volts. A battery fully discharged at 10.5 volts will rise to 13.5 volts - a voltage the charger will see as near full.

Solution - charge at a more sane rate. c/10 is usually a good choice.

#### sharkai

Joined Jul 29, 2016
5
No kidding. A battery has an internal resistance - and a discharged battery can have a significant internal resistance. When you input a charging current to that battery, a voltage is developed across the internal resistance, and the batteries terminal voltage will rise without any actual charging having occurred. When you charge at such and excessive rate, the large drop across the internal resistance will look to the charger like the battery is fully charge and the charger may very well drop to a maintenance rate.

Example with an battery internal resistance of 0.1 ohms, a 30 amp charge will cause the terminal voltage to rise 3 volts. A battery fully discharged at 10.5 volts will rise to 13.5 volts - a voltage the charger will see as near full.

Solution - charge at a more sane rate. c/10 is usually a good choice.
this is probably what is happening. That would explain the fake voltage thing, forcing the charger to slow down.
But we can't allow it to slow down.
The system attached in front of the batteries requires that much power. And the charging time is really limited. That is why fast charging is required
Is there some other solution.

Your example was awfully detailed for not being a real battery. Since you mentioned solar, I assumed (perhaps falsely) that they were lead acid batteries.

Can we move on to a real situation?
1) What battery chemistry are you pondering?
2) How many cells of that chemistry?
3) What is the usable watt capacity of the batteries?
4) What is the maximum charge rate?

John
Don't got all the other information but
3 x lead acid batteries = 36 V
At discharge state, voltage is around 30V, at which the system shuts down.
The batteries are little old but should be around 80% watt capacity.
Max charge rate is 20A from one charger and 10A from another charger.
Yes, he tried to solve the problem by attaching two chargers.

I think the fake voltage problem is probably what @Ylli stated, huge amperage caused huge voltage to show across the internal resistance of the battery.
But the battery need to be charged very fast. So what could be the solution?
So I hope the picture is little bit more clear.

Many thanks to @Ylli

#### #12

Joined Nov 30, 2010
18,217
But the battery need to be charged very fast. So what could be the solution?
More batteries in parallel. If you have 300 amp-hours worth of batteries, the C/10 rate is 30 amps. It's still going to take ten hours to charge at that rate, but the internal resistance isn't going to fool the charger into shutting down.

#### nsaspook

Joined Aug 27, 2009
8,263
I think the fake voltage problem is probably what @Ylli stated, huge amperage caused huge voltage to show across the internal resistance of the battery.
But the battery need to be charged very fast. So what could be the solution?
So I hope the picture is little bit more clear.
You left out a very important piece of information.
What is the Ah capacity for each battery in the series string?

With good flooded lead acid traction batteries recharging at 1/5 (C/5) of the battery capacity in Ah is easily done until about 85 percent state of charge (SOC). I keep my system at C/10 and don't discharge below 50 percent SOC to extend battery cycle life. As others have pointed out low-resistance and high charge acceptance lead/acid batteries (and circuits) is an absolute requirement for fast charging.

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