External Charger on PCB v1.2; Also, Due config?

[fghorow@gmail_com]

Hi All,

I'm finally stuffing/soldering my board, and I have a question about internal vs. external charging.

My battery is a 25.6V 6.6Ah LiFePO4 pack, with an onboard Battery Management System and a matching external charger.

For those in the U.S., the specific pack I bought is here , and the charger is this one . That battery pack fits in the chassis kit aft of one of the driver motors and to the side of the blade motor and its bracket. It was a fairly long search to find a pre-built pack that fits, and that _should_ give me about 2 hours or running time -- at 85 Watts -- for my way, way too big lawn. Note that the battery pack is considered "hazardous material" and needs to be shipped by ground...

On the v1.2 PCB, I'm unsure about which (if any) of R9 through R13 (for the voltage divider bridge) and any associated other discrete components need to be on the board to fit my situation using an external charger.

Also, In order to run the fancy (Java based) ~cm level GPS algorithm I discussed in another thread, I'm using a Udoo quad -- which is supposed to be "compatible" with the Arduino Due. In particular, it needs to be fed 3.3V instead of 5V. It appears from the limited understanding I could gather via Google Translate of the German discussion, the V1.2 board is able to be configured to run the Arduino Due at 3.3V, but I am not sure _how_ to actually do that on the board.

Any advice would be greatly appreciated!

Many thanks,
Frank Horowitz

 

[fredericg]

Hi Frank,

So, you plan to install the charger in the charging station? This measn that the charger will beconnected to the battery over the sliding contacts? I would be a bit afraid of this, as these contacts will we be far from perfect which means that the charger might have trouble accurateley measuring the battery charge state. IMHO, it would be better if the charge where also in the mower, but then the sliding contacts would have to carry 110V Ac...

Frederic

 

[fghorow@gmail_com]

Good point, Frederic. I hadn't really thought about the connection too much.

I suppose I could always try to deal with that via brushes for the electrical contacts on the dock side of the rig.

An actively sliding contact surface is probably making/breaking electrical contacts on a ~ microsecond timescale (due to sliding asperities). Any stray (or designed) capacitance in the circuit is probably enough to hold the DC part more-or-less constant.

The situation here is different. The contact is static once the mower stops moving. The situation is more like the contact surface of a switch than that of a sliding brush. There might well be a variable contact resistance between different docking circumstances, depending on the details of the configuration of the two electrical contacts.

This must be an OLD problem (and hence throughly studied) in electrical engineering. (Well before the days of electronic engineering.) Do we have any EEs in this group that know anything about this problem, and who can recommend ways to deal with problems? (Surface coatings with graphite or something springs to my mind, but there must be a tried and true solution to the switching contact resistance problem...)

 

[jack0]

Hello

I'm also building a mower with a 25.6V 6.3 Ah LiFePO4 pack, assembled by myself, with an onboard Battery Management System (http://www.eclipsebikes.com/lifepo4-288v-cell-30a60a-p-1040.html) and a matching external charger ( http://www.eclipsebikes.com/lifepo4-battery-charger-p-1036.html).
The BMS is in the mower, as it balances current between the cells when loading and also when discharging the battery. This is supposed to give a better efficiency and a longer life to the battery. As both the BMS and the charger already include all protections (under and over voltage, short circuit), I think all the loading part of the PCB 1.2 is not necessary anymore. So I did not install r9, r10, r11, r17, r18, r19, also the relay and the transistor, D1, D3, D4, D5, D6, D10, C1 and C4, INA169 modules, jumper JP4 and JP6 and 5 A fuse. So I only connect the output of the BMS (see attached BMS schematic) to the battery connector on the PCB. And I had to add a bridge between input and output of U5 module for the DC/DC converters to get 24 V supply.

Furthermore, I do not want battery tension to remain on the loading contacts when not in the station, for safety purpose and to avoid accidental discharching or short circuit by some vicious wet tree leaves falling just on the wrong place :angry: . So I add a small circuit (see LoadingRelay schematic) between the loading contacts and the BMS. It also protect the mower in case of poles inversion when connecting to the station. Short description : if a tension is applied between the loading contacts, as in the loading station, the relay closes, the BMS connects to the contacts and the led ligths. If reverse polarity, the diode will avoid the supply of the coil, hence nothing appends. If there is no tension between the loading contacts, the relay is open and the BMS is disconnected from the loading contacts.

In the charging station the mower presence is detected by a switch. Through a relay, it closes the 24V for loading the battery and disconnect the supply of the peripherical wire.

Jacques
Attachment: https://forum.ardumower.de/data/media/kunena/attachments/2809/8CellBMSinstall.jpg/