Parallel batteries (again!)

[AntonioAlfaro]

Support information on internal resitance.

Test of Sanyo NCR18650GA 3500mAh (Red)

 

[matthewslack]

Let's go one step further in our model of DIY extender.
We have already seen that we can connect two "same" battery banks in parallel as long as both banks have the same charge. How charge and voltage are related according to the cell manufacturer's curves. So we can say "equal volages = equal charges".
Now, if bank A has a higher charge than bank B, when they are connected in parallel, charges from A will pass to B through an electric current limited only by the internal resistance of the battery banks. these internal resistances are of the order of milliohms and that is why large currents would be caused depending on the level of charge imbalance of bank A and B. Destruction could occur. For this reason, battery chargers limit the maximum current delivered to the load.
Here comes the first alert with the DIY extender.
In our lab-scale model, we connected two banks with different charge levels in parallel and measured the current that was produced. See attached figures.

Bank A = 83% charge volt=8.10 v
Bank B = 72% charge volt=7.94 v
Charge imbalance = 15%
At paralleling , I = 44.4 mA from A to B charging rate 0.02C
After some minuts , I = 30.7 mA from A to B charging rate 0.01C
If we let the banks be connect , they will equalize their charges.

One thing you should pay attention to is that the basic cell's voltage curve is almost flat before reaching the point of discharge ( from 4.2v to 3.0v). In the DIY extender and main batt ( from 42v to 30 v).
So small voltage differences between bank A and B can mean large charge differences. Don't forget this point.
I will continue...

The internal resistance seems very high: voltage difference divided by current is resistance round the circuit I think, and unless there is something else in it, will be almost entirely the internal resistance of the two batteries.

(8.11 - 7.91) / 0.0444 = 4.73 ohms, or 1.18 ohms per cell. The data sheet indicates 0.1 ohms per cell, so is there a factor of 10 error creeping in somewhere?

Something doesn't look right here.

 

[Nealh]

Panny's/sanyo tend to have a higher initial IR.

 

[AntonioAlfaro]

The internal resistance seems very high: voltage difference divided by current is resistance round the circuit I think, and unless there is something else in it, will be almost entirely the internal resistance of the two batteries.

(8.11 - 7.91) / 0.0444 = 4.73 ohms, or 1.18 ohms per cell. The data sheet indicates 0.1 ohms per cell, so is there a factor of 10 error creeping in somewhere?

Something doesn't look right here.

What you write down is correct. I would say that it is not the expected result. But it is real for that test fix. The only explanation I have is that these banks are not new and have too many charge and discharge cycles.
We will leave this test phase until there and we will go on to tests with the real benches that we will use in the DIY extender.

 

[AntonioAlfaro]

Next I will show a series of images about the construction process of the DIY extender. A series of manual modification work was necessary but nothing that cannot be achieved with patience and care.

 

[AntonioAlfaro]

The following image shows a cheap and useful instrument to which I have put a connector compatible with the ones we will use in the DIY extender. We can measure voltage and current simultaneously.

 

[AntonioAlfaro]

Here we can see the adaptations and connectorization for the DIY extender.
M2 is an adapter to charge the DIY extender batt with the same Mahle charger that we use for the main batt. This is optional.

 

[AntonioAlfaro]

The first basic measurement we will make in the actual DIY extender and main batt bank will be to charge
separately up to 100% (when the charging current is < 20 mA the ammeter used will mark o mA) both batteries.
After completing these charges up to 100% on both batteries we will take note of the OCV open circuit output voltages of both battery banks.
This record is extremely important.
Main OCV = 41.9 v
DIY extender OCV = 41.4 v
OCV difference = 0.5 v
It is important to keep in mind that we will never find the same OCV. But more important is to always know the voltage difference for our battery banks because they will change as the charge cycles increase.

Knowing the OCV differential we can begin to draw basic conclusions.
I will continue..

 

[AntonioAlfaro]

DIY Extender already finished and installed.
After 44 Km on road test,
The behavior of the currents and voltages of the Extender and Main Batt, were equal to the measurements in the static test with route simulation in Rouvy and using the Wahoo smart trainer, therefore the project is finished.
For additionalinformation please send me PM.

 

[Yak]

The behavior of the currents and voltages of the Extender and Main Batt, were equal to the measurements in the static test with route simulation in Rouvy and using the Wahoo smart trainer, therefore the project is finished.

So, Antonio, have you concluded whether the extender is working as a charger?

 

[AntonioAlfaro]

Hi Yak, more than that. The mistery now is complete clear. Diy Extender has almost 90km road real test and I am putting all my tests and results in a report.

 

[Yak]

Hi Yak, more than that. The mistery now is complete clear. Diy Extender has almost 90km road real test and I am putting all my tests and results in a report.

Looking forward to it.

 

[AntonioAlfaro]

Looking forward to it.

Yako, send me your email address.

 

[Sturmey]

The internal resistance seems very high: voltage difference divided by current is resistance round the circuit I think, and unless there is something else in it, will be almost entirely the internal resistance of the two batteries.

(8.11 - 7.91) / 0.0444 = 4.73 ohms, or 1.18 ohms per cell. The data sheet indicates 0.1 ohms per cell, so is there a factor of 10 error creeping in somewhere?

Something doesn't look right here.

Hi. How did you measure the current. The meter itself can introduce an error when reading current at such small voltage differences and you need to take into account the resistance of the measuring device, leads and probes at the scale that you were using. For example, some Fluke meters introduce a 'Burden' of well over an ohm on the current scale.

Can you live with the burden?

One of the realities of electrical testing is that you can't measure something without changing it in some way.

www.fluke.com

 

[matthewslack]

Hi. How did you measure the current. The meter itself can introduce an error when reading current at such small voltage differences and you need to take into account the resistance of the measuring device, leads and probes at the scale that you were using. For example, some Fluke meters introduce a 'Burden' of well over an ohm on the current scale.

Can you live with the burden?

One of the realities of electrical testing is that you can't measure something without changing it in some way.

www.fluke.com

I was commenting on the measurements in a previous post, not making my own. It may well be down to the instruments used. I've not got down to that level of detailed analysis in my experiments!

 

[AntonyC]

Hi Chamzamzoo, have you come across this, it might do what you want:

Pardon our interruption...

The extension battery feeds this regulator which you adjust to give up to 2A at up to the fully charged voltage of the original battery. Then to the original battery it looks like a charger and supplies the first 2A of {charging current + motor load}, if any.

Unlike the 'batteries in parallel' method no mods are needed to the original battery, no diode in other words, which I'd guess is why the bike's makers chose the 'extender' route.

The battery voltages needn't be matched, the extender battery's voltage can safely be higher, or it can be lower and will be boosted as needed (up to a point).

This does rely on the external battery cells having a generous current rating, and on its LVC protection, and on the original BMS working with an extender (tick). You should gain about 80% of the extender pack's capacity.

I'd fit a diode in the regulator's output, just in case, and choosing similar battery voltages should make for a stable, efficient regulator. I've not used this one myself but maybe it's worth a punt.

Anthony

 

[AntonioAlfaro]

Hello,
I am attaching a draft of the test report that I carried out on my the DIY Extender.

 

[nigelbb]

Hi Chamzamzoo, have you come across this, it might do what you want:

Pardon our interruption...

The extension battery feeds this regulator which you adjust to give up to 2A at up to the fully charged voltage of the original battery. Then to the original battery it looks like a charger and supplies the first 2A of {charging current + motor load}, if any.

Unlike the 'batteries in parallel' method no mods are needed to the original battery, no diode in other words, which I'd guess is why the bike's makers chose the 'extender' route.

The battery voltages needn't be matched, the extender battery's voltage can safely be higher, or it can be lower and will be boosted as needed (up to a point).

This does rely on the external battery cells having a generous current rating, and on its LVC protection, and on the original BMS working with an extender (tick). You should gain about 80% of the extender pack's capacity.

I'd fit a diode in the regulator's output, just in case, and choosing similar battery voltages should make for a stable, efficient regulator. I've not used this one myself but maybe it's worth a punt.

Anthony

I have a battery case for my iPhone XS which work in this way. The battery in the phone is kept charged at 100% by the battery in the case.

 

[Yak]

Hello,
I am attaching a draft of the test report that I carried out on my the DIY Extender.

Excelente Antonio. You have found that the extender provides more than the 2a charging current when the motor is drawing more than 4a under load. This is what I had hoped. But would this be possible if the extender acts as a passive charger, and as the main battery is depleted more quickly when under +4amp load, that the extender is simply putting more charging current into the main battery? Have you tried to establish whether the extender will equalise slowly (‘charge’) the main battery if the extender voltage is the higher of the two, or whether, as Volabike claim, current is taken solely from the extender under load until the two batteries are at equal voltage?

 

[AntonioAlfaro]

Hi Yako,
I have readings where the motor is drawing 7 amps and the extender is providing more than 3 amps.
In my opinion (based on measurements and not assumptions) the DIY Extender works in parallel with the main battery once the extender is equalized with the main. (this happens very quickly). The DIY extender does not work as a charger for the main one. They work by providing the same amount of amps, that is, they share the load equally. That means both batteries discharge at the same rate.