Roy's StuG III (no. 2020/10)

[Adrian Harris]

Charging in series is directly affected by the internal resistance of the battery - good old Ohm's law.

The voltage across each battery is (V x Rx)/(Rx + Ry) and (V x Ry)/(Rx + Ry), where V is the charging voltage and Rx and Ry are the internal resistances of the batteries.

If the charging voltage is 28.8V and the internal resistance of Rx and Ry are both 2 milli-ohms, then the voltage across each is the same and 14.4V.

As batteries age, the internal resistance increases.

If Ry increases by 1%, then the voltages are 14.33V and 14.47V.
If Ry increases by 5%, then the voltages are 14.05V and 14.75V.

It's a vicious circle, in that while the battery charger sees the combined voltage, battery X isn't getting a fair share. So it doesn't get fully charged and hence depletes quicker when in use.

Also battery Y will want the charger to turn off when it has charged up to the correct voltage, but the charger will keep going until it sees the right voltage across the pair, hence overcharging battery Y.

Overcharging an AGM battery will shorten its life, so now you have a pair of batteries where one is being under charged and the other is being cooked. A perfect storm.

Adrian.

[michael hilton]

Thank you Adrian, clear and concise. Love 'perfect storm' ......Jab day tomorrow...can't wait....Mick

[Roy Beukeveld]

Hi Adrian,

Thank you for your explanation, Ohm's law yes indeed
You are absolutely right, i obviously hadn't looked at it this way.

The only right thing to do here is charge separately, so i will wire in a selector switch in the charging leads.
That spare switch in the right panel comes in conveniently now

[Tom OBrien]

Adrian,

A great explanation and simple enough for me to understand.

I'm curious. some chargers advertise "balanced charging", where (I assume) the charger can detect when one of several batteries wired in series is undercharged and the others are overcharged. I further assume that when detecting an imbalance, the charger can compensate in sending more current into one over the other. Does that make sense?

Roy: Your build is amazing. I wish I had the means to fabricate the electronic mountings you've made. It's a modeler's model.

cheers... Tom

[Roy Beukeveld]

Hi Tom

Looking back on my LiPo's, they were all balanced. A battery pack with 6 cells had two main (charge) leads, and six small wires combined into one connector. The balancing connector. When charging this battery the charger measures the individual cells. Over/under charging should therefor not be possible. When the charge cycle was complete all cells were nicely balanced out with a minimum of difference.

Thank you for the kind words on the mountings. Designing them is most of the work, for the rest the printer deserves most of the credit
I have had this machine for quite a while now, it's amazing how many applications you find for it over the years.

[Adrian Harris]

Chargers for LiPo batteries have a connection to every cell, so that they can monitor the individual voltages of each and regulate the charge going to them. "Balancing" a LiPo is the process of bringing voltages of all the cells to the same value, so that as the battery discharges, each cell should discharge at the same rate. If one were to start of at a lower voltage, you might take it beyond the recovery point before the overall battery voltage has dropped to the lower limit.

On LiFePO4 batteries this is usually done internally by the BMS, or it can be done with a decent charger if the battery doesn't have a BMS.

Lead-Acid and AGM/Gel batteries only have two connections, so there's no way for the charger to detect the voltage across each cell.

Adrian.

[Roy Beukeveld]

Hi everyone,

It's been a bit quiet here on my StuG build, spent quit a lot of time working on the wiring.
The two panels I made beneath the engine deck hatches require a lot of wiring, and to keep it nicely organised was a bit of work.



When you take of the engine deck you also want some length in the cables so you can set the deck aside. The length and the several interconnections made it a bit messy, and hard to change it in the future if needed, or to troubleshoot problems.
So I made a junction box with numbered pins for the connections. And of course made a proper mount for it in the hull.





I also mounted the speaker, and made a protection guard for it. I mounted it to hull side, and it rests on the battery. It situates nicely underneath the large hatch. The speaker can be hinged upwards if you want to take the batteries out.







So leaving that for what it is now, I'm continuing on the superstructure. The climate in the garage is improving so back to building and painting

[Adrian Harris]

Well that must be the neatest wiring I've ever seen in an Armortek model

Are you printing in PLA, ABS or something else ?

Adrian.

[Robert E Morey]

Roy your wiring is awesome! I really like your junction box. Is that home design? I've been looking for commercial power block without much luck. Most of them I can find have huge thread terminals - overkill for tank model. Yours is really nice and compact. Love your speaker set up as well. Great ideas! Much better than the spaghetti inside of my tanks!

[Vince Cutajar]

spent quit a lot of time working on the wiring.

You can say that again. Very neat layout.

Vince

[Roy Beukeveld]

...Are you printing in PLA, ABS or something else ?...

Hi Adrian, thank you very much for the compliment, the prints are made out of Ultrafuse PLA, I have been using this brand of filament for quit some time now and I can surely recommend it.

...I really like your junction box. Is that home design? ...

Hi Robert, thank you! Yes it is home design. The two rows you can see have also a second one laying beneath it. So every pin has a 'A' and 'B' connection. The lower connections are used for the pin interconnections, and the upper ones mainly for the entry of the wires.

...You can say that again. Very neat layout. ...

Thanks a lot Vince

[Robert E Morey]

Hi Roy, regarding your box. What do you use for the metal bus bar? I assume there's a metal strip inside there somewhere to transfer the electricity to all terminals? Sure is a cool set up.
B

[Roy Beukeveld]

... What do you use for the metal bus bar? I assume there's a metal strip inside there somewhere to transfer the electricity to all terminals? ...

Hi Robert,

Well it is a bit different in this case, there is no metal strip connecting all the pins because the schematics are different.
I made a cross section screenshot from CAD to show how the pins work.



Interconnections are wired from pin to pin where needed. Not every pin is powered though. For instance, I also have two pins which function as a switch. Regardless of voltage. Operated by one of the flip switches in the panel. This one for instance is used to switch the smoke unit on and off which has it's own power supply.

The large pins connect 4mm2 cable to M6, the small pins connect 0,5mm2 and 1,5mm2 to M3.

[Christoffer Ahlfors]

Spaceship tidiness!

You, sir, are very inspiring! First, you made the beautiful panels, which inspired me to do something for my charging connectors. Now you raise the bar and show us that the interior is just as tidy. It will take me some time to digest this!

Thank you so much for the leadership and inspiration!

[Roy Beukeveld]

Hi Christoffer, thank you very much for your kind words! Glad to hear my creations inspire and generate new ideas