Battery Swapper 56v

Other Hardware Hardware Development
1

I am looking for some ideas for a project I need to put together.

I have a device that uses 56v batteries and I need the ability to automaically "swap" the batteries during the discharge and charge cycles.
The batteries have a charger so I don't want to replace that or have to deal with any of the issues with battery management.

The scenario I am thinking of is
Discharge Scenario

  • Arduino monitors the charge level of all batteries
  • The device runs from a single battery at a time
  • When a battery gets to 10-20% it will swap to the next battery in the line

Charge scenario

  • The existing battery charger is connected
  • The system will connect one battery at a time direct to the existing charger. This allows the battery charger to do its thing.
    If the system messes up and leaves a battery connected it won't matter because the battery charger handles this state.
  • Once the charge of the battery is full, it will switch to the next line in the battery, going through each until it is fully charged.
  • I am able to get the battery charging % from the charging unit itself to the Arduino.

The reason for "swapping" instead of wiring them together is due to battery management of the charging side.

I was looking at using a simple voltage divider to measure the voltage of each battery to determine the discharge.
But I understand Lithium batteries don't have a voltage curve to determine an accurate percent. But since I really only need to know full or almost empty, do people think this would work in this scenario?
Or is there a better solution? I did look into Coulomb Counters but I can't find anything that would work within the 56v range.
Just been reading up on the ACS712 Hall effect current sensor. Im thinking this maybe an alternative but I have never used one of these.

I have already had a bit of a play around with a voltage divider that will allow me to turn on and off the battery measurement and im getting voltage readings into an Arduino.

For the switching side, when charging im looking at an Arduino switch a transistor which will in turn switch a mosfet for the positive side of the battery and two SSRs for the battery data lines. This would be for the charge side.

For the load side, the Arduino would switch a single mosfet to connect the positive to the load.
All the negative sides would be tied together and to the Arduino ground.

Im keen for any thoughts, anything I have missed out. Let me know if I have also mised out any important detail that will help with feedback.

Thanks.

2

What is the maximum current this device will draw.

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I've used Hall effect current sensors are I am very dissatisfied with them because of their noise. I used a lot of hardware and software smoothing and averaging but was never satisfied with the results.

Does your circuit use positive or negative ground? The answer affects how you go about this. Please post a schematic showing how you think it could work, hand drawn and photographed is fine.

Beware of using transistors of any kind to switch battery connections as the current has to flow both ways (charging and discharging), something people forget with this kind of thing, and as a result they design circuits that can't work because transistors only work one way round.

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What device are you powering with this bank of batteries ?

5

You could use a DPDT (Double Pole Double Throw) relay to switch batteries. That would isolate the one on the charger.

6

56v @ 13a

Good point. I will consider relays again.

I was unable to find any relays that would work at 56v DC 13a.
They all seem to max out at around 32v DC.

I did just find this which would probably do https://www.digikey.com.au/en/products/detail/american-zettler/AZDC007-1AE-12D/22050421
Price isn't ideal though and no stock. SSR is even more expensive. They look like around USD$30 each and ill have a direction flow issue.

Could I just get away with using this to switch the +ve of each battery and leave the -ve connected in both load and charge states?
Can i then also use one relay to switch one side of the load and charge?
There will be 5 batteries + the load + the charging.

7

The coil voltage can be whatever you can get, it is unrelated to the contacts other then it has to move them but no electrical relationship. Here is a picture of one like I have used in the Past. I do not know this brand but they look the same.
image

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I was looking at the ratings for the contacts.
As from what I understand is if I don't get ones rated high enough it won't be able to break the current flow, especially when using AC relays in DC.

That one i was looking at is

Coil Voltage - 12VDC
Contact Rating (Current) - 50 A
Switching Voltage - 65VDC - Max
Load - Max Switching 3250W

But super expensive.

That one you linked appears to be this one?
https://au.element14.com/schneider-electric-legacy-relay/199ax-9/relay-dpst-no-300vac-28vdc-40a/dp/1566672

Contact max 28v so even that I don't think would be suitable.

9

You may be able to get two contactors for less. When you connect them use the aux contact to lock out the unused one.

10

Definite purpose (DP) contactors (used in HVAC ) usually have quite a high VDC break ratings. Especially if you are buying 600VAC capable contactors. They are not always labelled so but if are rated for 600VAC can almost guaranteed break
125 - 250 VDC.

https://www.amazon.ca/gp/aw/d/B07C1YQB6R?psc=1&ref=ppx_pop_mob_b_asin_title

You are getting into very specific territory when you get past 48vdc unfortunately. Omron also has great DC break power relays. But cost…

11

It seems I failed at searching properly and there were some other relays that appear suitable.
I have ordered some relays and other bits for some quick prototyping.

I will continue with the voltage divider and the hall effect to see which is better for this application.
I don't need to show % charge, so as long as I can roughly cut out the batteries in the 10-20% range, it should be fine.

Thanks everyone for the assistance. I will post a schematic for feedback when I am able to get a mockup done.

2 Likes
12

A very rough sketch. My first time using KiCad so the lines are a bit of a mess.
Im sure I probably have some things in the wrong place.
I still need to add in the hall effect current sensor and the indicator lines from the charger.
However, I am interested in any initial feedback of if something like this would be workable?

image
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1 Like
13

I have cleaned up the sketch if anyone has any thoughts?

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14

Great job, I do not think my fifth was that good.

15

Nice, neat schematic, well done!
For your next attempt you can improve it by following the rule of positive voltages towards the top, negative towards the bottom, signals in on the left and out on the right.

Fuses and batteries
You have fuses for the batteries next to the relay contacts; the main reason for using fuses is to protect the wiring from the high currents possible if there is a short circuit. All but the smallest batteries can produce sufficient current into a short circuit to heat wires enough to start a fire, this is particularly true of lead acid and lithium batteries. Placing the fuses where you have them does not protect the wiring between the board and the battery. I would place the fuses as physically close to the batteries as possible. For a multi-cell battery I would place the fuse in one of the wires linking the cells of the battery.

The schematic does not show the batteries themselves, so I can't see what is going on there. What are the middle 2 connections for and how are they connected? I see they go to the 4066s. The 4066s are powered from 5V, which means they must not have a voltage outside the range of 0V and 5V applied to any of their inputs. As I can't see where these connections go I can't see if this is the case, but given that they are 56V batteries I suspect there will be more than 5V on 4066 inputs.

Although the BJTs you have operating the relays will work perfectly well my preference would be MOSFETs.

The 470Ω resistors for the LEDs will give about 20mA through the LEDs, I would guess this will make the LEDs a bit too bright, it might be worth experimenting.

Voltage divider:
IRLZ14 is an N channel MOSFET. An N channel MOSFET conducts when the gate voltage is sufficiently positive of the source to turn it on. How much positive depends on the actual MOSFET. In the case of the IRLZ14 it has an on resistance of 0.2Ω with Vgs = 5V. In your scematic you have the source connected to +56V and 0V or 5V on the gate, it is never going to turn on. If you are switching a positive voltage you need a P channel MOSFET combined with either an N channel MOSFET or an NPN transistor to control the gate. I'm not going to propose a circuit for you, this kind of thing comes up a lot and there are examples in this forum and all over the internet.

I don't know what the 100nF capacitor on the gate of the MOSFET is for: it will just make the MOSFET switch more slowly, not withstanding that it won't switch on at all, see above comment.

I don't know what the 1n47xxA diode in series with the gate is for; all I can see it doing is reducing the drive voltage to the gate and thus slightly increasing the on resistance.

Your voltage divider has 100k in series with 1.5Ω, giving an output of 0.00084V, which you are not going to be able to measure.

Serial port
You don't normally need a 5V connection for a serial port, Tx, Rx and ground are usually all you need.

Decoupling capacitors
While I can see some you also need them close to each 4066, and anywhere else I might have missed where something is fed from 5V. If in doubt add another decoupling capacitor, there is no such thing as too many decoupling capacitors! (Awaits someone making a cartoon of a board covered in decoupling capacitors).

Relays
I think this might have been covered elsewhere, but relays capable of switching 56VDC reliably are not common and not cheap. Interrupting DC at anything above about 24V is difficult with mechanical contacts because the arc that starts when the contacts part does not self extinguish. With AC the current stops 100 times per second (in the UK), which interrupts the arc. No such thing happens with DC.

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Thanks for the feedback. I will work on those changes.

You have fuses for the batteries next to the relay contacts; the main reason for using fuses is to protect the wiring from the high currents possible if there is a short circuit

Thanks I will move the fuses

The schematic does not show the batteries themselves, so I can't see what is going on there. What are the middle 2 connections for and how are they connected?

The batteries are EGO 56v 10Ah. They are sensor lines. The battery will discharge without these being connected but the battery charge indicator does not update. Also some devices require these to be connected to operate. Eg if I don't have the sensor lines connected on a leaf blower, it kicks in for 1 second then stops.
I also need these lines for when it operates in reverse so the charger can know the state of the batteries.
I don' want to mess with the charging or discharging, the whole point of the board is to allow the charger and loads to operate normally just by "swapping" the batteries out with the relays.

Voltage divider

Yeah I need to redraw this section. I have a working version on a breadboard but I realised I copied it over to the schematic wrong in my initial design, because it was a mess.
Being lithium batteries I don't think the voltage divider will be of much use anyway because of the drop off curve but I will keep it in for testing.

You don't normally need a 5V connection for a serial port, Tx, Rx and ground are usually all you need.

I am using an external serial to usb controller. It is just matching up the lines required on the board. These will only be used for development

Decoupling capacitors

Good point I will add these

Relays
Yep I found some that work and they aren't too expensive, ~$3 each.
They can do 100vdc @ 30a. https://www.digikey.com.au/en/products/detail/picker-components/PTRH-1A-12SF-X/12318044

Thanks again.

18

This tells me very little. My concern is that the voltage on them might be outside the 0V to 5V that can safely be applied to the 4066s, you need to check this and confirm the voltage range on these connections.

As for the rest, when you have a new schematic please post it. It might be worth waiting a little while to see if anyone else has any comments that might change things.

19

I think it’s single wire serial 3v

Edit:

I don't have an oscilloscope and couldn't get a multimeter reading. So I thought I would risk a 4066.
It works perfectly in connecting the data line.

20

Updated version

  • Fuses removed. I'll add these to the battery terminals
  • Caps added to the 4066s
  • Cap added to output of the 12v supply
  • Fan connection added
  • Voltage divider redrawn

The idea of the voltage divider was to only measure the voltage I request the voltage. However I just calculated that it would probably take around 800 days for this to drain the battery, so I think I just leave out the switch to keep it simpler.

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