# Cheap Series BMS



## lnpurnell (Sep 27, 2012)

Hi all,

Quick question,

I have a 15 cell LiFePo4 pack that I want to monitor either the individual cell levels or overall pack voltage (I am going to bottom balance)

I do not want the BMS to cause my cells to drift at all. Is there a BMS that can be used that does not cause this? 

I was thinking a cheap series BMS that scans though the cells in sequence.

Also, is there a simple High Voltage and Low voltage cut-off relay that I can use when hitting the extremes of my pack? I do not mind manually resetting the relay afterwards.

Thanks,

Leigh


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## lnpurnell (Sep 27, 2012)

I have been looking into Cellog8 monitoring, does anyone have any recommendations?

- What sort of prices does a Celllog8 system go for?
- Does it create an unbalance in the pack?
- Do they include relays for LVC and HVC?

Thanks!


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## PStechPaul (May 1, 2012)

I made a voltage relay that can easily sense the pack voltage and trigger an alarm or shut down the system if it goes above or below two setpoints. See:
http://www.diyelectriccar.com/forums/showthread.php?t=84319&highlight=voltage+relay

I made another version that operates at a much lower voltage, upon request from a member of this forum. The parts are about $20, but it takes e a couple of hours to build so I'd have to charge considerably more. However, you can build it on a piece of proto-board as I did. The schematic is in the thread above and I can supply the PIC assembly code. I also have most of the parts needed. Contact me if interested.


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## dougingraham (Jul 26, 2011)

lnpurnell said:


> Also, is there a simple High Voltage and Low voltage cut-off relay that I can use when hitting the extremes of my pack? I do not mind manually resetting the relay afterwards.


Simple and cheap BMS's are going to eventually unbalance the pack. The terms simple and cheap make this a forgone conclusion. It would take a very long time to unbalance the pack if all you did was buy 15 DVM's and connect each one to a cell. As long as it doesn't take its power from the cell it is measuring the imbalance should be tough to measure in the life of the cell.

For High voltage cut you could use a JLD404 meter to turn off the charger or force a contactor open.

The limit on the bottom end isn't something that you usually need to worry about. I set mine to 1/2 nominal pack voltage because in the winter the cell sag is so great when it is cold. This is 1.6 volts per cell or 24 volts for your 15 cell pack. If it isn't cold you will probably never get there until the pack is exhausted. Most of the motor controllers have this as a configurable parameter.


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## lnpurnell (Sep 27, 2012)

PStechPaul said:


> I made a voltage relay that can easily sense the pack voltage and trigger an alarm or shut down the system if it goes above or below two setpoints. See:
> http://www.diyelectriccar.com/forums/showthread.php?t=84319&highlight=voltage+relay
> 
> I made another version that operates at a much lower voltage, upon request from a member of this forum. The parts are about $20, but it takes e a couple of hours to build so I'd have to charge considerably more. However, you can build it on a piece of proto-board as I did. The schematic is in the thread above and I can supply the PIC assembly code. I also have most of the parts needed. Contact me if interested.



That looks great Paul!

Can this be used off the 48v pack so there won't be any individual cell sag?

I am looking for a voltage relay with a voltage indicator, a manual button to display voltage (to reduce power draw) and a manual reset button for the relay.

Can all these be placed in series so your voltage relay can be triggered cutting all loads to the pack?

I see you diagram displays diodes. Does this mean that the pack can be charged through the same circuit, even with the relays open?

Thanks!

Leigh.


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## PStechPaul (May 1, 2012)

The schematic in the other thread was a conceptual design using LTSpice. Here is the schematic for the 16VDC version, which does not use the diode on the input:
http://enginuitysystems.com/files/VRLY_16V_PCB_Schematic.pdf

Here is the complete schematic for the 80-240 VAC version. Some parts were changed or eliminated for the 16VDC version.










I'm not sure exactly what you mean about the diode and using it for charging. You can put this across the entire 48V pack and the relay can be made to stay closed within a range of acceptable voltages, such as 37V (2.5 Vpc) and 54V (3.6 Vpc). It could also have a time delay at the low end to handle short time sag under heavy load. And it can be made to lock out so that you need to disconnect it for reset. There are three spare I/O pins which might be used for a pushbutton, or possibly a display. I recently bought some LCD display modules (for $0.99) with 12 7-segment digits that use 4-6 I/O lines, and there are also SPI modules which can control standard parallel dot matrix LCD modules using 2-3 pin SPI or I2C. 
http://www.ebay.com/itm/170968157104?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Almost anything CAN be done, but this particular design is limited. It could be connected across the entire 48V pack while either driving or charging, but it draws about 10-50 mA so it could not be used when the pack is idle. If you need individual cell voltages, you really need a proper BMS, which will cost about $5-$20 per cell. I am working on a design but the first version will be for 12V SLA batteries. It could be made also for LiFePO4 or other chemistries, and the current drain could be made to be low enough to keep on the cells indefinitely. It could even have individual LCD displays which draw only a few microamps, but that would add a few dollars to the cost and would be of limited use unless you can see the batteries to which they are connected. 

Since my own needs and desires may be unique, it would be best to put together a full specification including necessary performance as well as a "wish list". Also remember that there is a principle where it can be made cheap, fast, or good. Pick any two. And sometimes you can only really do one.


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## lnpurnell (Sep 27, 2012)

PStechPaul said:


> The schematic in the other thread was a conceptual design using LTSpice. Here is the schematic for the 16VDC version, which does not use the diode on the input:
> http://enginuitysystems.com/files/VRLY_16V_PCB_Schematic.pdf
> 
> Here is the complete schematic for the 80-240 VAC version. Some parts were changed or eliminated for the 16VDC version.
> ...



Hi Paul, 

I have written a specification, let me know if you understand this.

Voltage Relay Specification list.

•	48v DC off the pack to power the voltage sensor relay (prevents individual cell sag)
•	LiFePo4 Battery Chemistry 15 cells (48 volts open circuit)
•	High Voltage cut-off at 52.5v (3.5v per cell)
•	Low Voltage cut-off at 45v (3v per cell)
•	A Normal open (NO) relay for the main supply.
•	A normally open push button switch for resetting the voltage relay when it trips.
•	30 second time delay after manual reset before tripping
•	The Voltage Sensor should be placed in series after the relay to turn off the sensor after the NO relay has tripped.
•	When the relay trips, the draw on the pack should be Zero.

The voltage sensor will draw the power from the whole pack when in use, the voltage sensor will power the (NO) relay to connect the battery pack to the rest of the system. 

When the pack voltage reaches i.e. 45v (low voltage cut-off) the (NO) relay will be de-energised and the relay will disconnect the circuit; the relay also powers the voltage sensor, so this too is turned off. 

There should now be no draw from the pack.

In order to reset the pack, a normally open push button switch has to be pressed in order to supply the voltage sensor with power. Once power has been returned to the voltage sensor the NO relay energises and connects the pack again.

The Voltage sensor relay will require a 30 second time delay before de energising the NO relay and powering down the pack. This is to enable the user to put the pack onto charge and prevent disconnect.

I hope this helps, let me know if this still does not make sense and I can attempt to draw a rough circuit diagram.


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## PStechPaul (May 1, 2012)

This should be quite easy to implement. Just a few modifications to the circuit will make it work within the 45-52.5 VDC limits. The relay I have chosen for this is 48 VDC and should work directly within those limits, but if not, a 24 VDC can be used with a regulator as shown. This relay has DPDT contacts and is rated at 8A 250VAC and 1A at 60VDC, and it's only about $3:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtSzCF3XBhmWwgemHWPRD5jlK1e8IZsROo=

The circuit could be powered through a NO pushbutton and a NO contact on the relay, so the circuit will be energized through the pushbutton and then latched on if the voltage is within the limits specified. The other set of contacts can be used to control the charger or the motor controller. There can be a 30 second time delay for tripping on undervoltage (or overvoltage), and it is also possible to require the reset button to be pressed for some time before resetting.

Something else that could be added is a mid-pack sensor which would determine if the voltage is exactly half the pack voltage, which is usually enough to see if the under or over voltage condition is caused by a cell imbalance. This would be easier if you had 16 or 14 cells. But a voltage divider could be made for 7+8, or a potentiometer could be used to adjust to the exact voltage of a healthy pack. The sense circuit would impose a load of about 25 uA on the lower half of the pack, but it could be switched off when the circuit is disconnected, and it may be possible to balance the current drain by adding another resistor across the top half of the pack.

This would function as a "poor man's" BMS with a parts cost of less than $10 for DIY and a low volume complete assembly for about $40.


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## lnpurnell (Sep 27, 2012)

PStechPaul said:


> This should be quite easy to implement. Just a few modifications to the circuit will make it work within the 45-52.5 VDC limits. The relay I have chosen for this is 48 VDC and should work directly within those limits, but if not, a 24 VDC can be used with a regulator as shown. This relay has DPDT contacts and is rated at 8A 250VAC and 1A at 60VDC, and it's only about $3:
> http://www.mouser.com/ProductDetail...=sGAEpiMZZMtSzCF3XBhmWwgemHWPRD5jlK1e8IZsROo=
> 
> The circuit could be powered through a NO pushbutton and a NO contact on the relay, so the circuit will be energized through the pushbutton and then latched on if the voltage is within the limits specified. The other set of contacts can be used to control the charger or the motor controller. There can be a 30 second time delay for tripping on undervoltage (or overvoltage), and it is also possible to require the reset button to be pressed for some time before resetting.
> ...


Hi Paul,

That sounds great, I wasn't sure if I was explaining it properly before, I am not too sure about adding a mid pack sensor as the issue of cell imbalance will rear its head again (even using a resistor). The idea of this relay will be to indicate there is a problem and just keep the cells out of the 'danger zone' *top gun pun intended* 

I would be happy to cover the costs for the parts etc, or if you could modify the schematic then and supply the PIC asssembly code that would be amazing!

Let me know 

Leigh.


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## PStechPaul (May 1, 2012)

Here is a simulation of the center tap balance monitor using 7 and 8 3V cells. The voltage at the two sense points S1 and S2 is 1.000V because of the chosen values for the resistors corresponding to the two half-pack voltages. The current out of each battery is exactly 1.000 mA.










If V2 drops to 23V, indicating one cell has dropped to 2V, the current through V2 becomes 968 uA and V1 becomes 989 uA. This may actually be good because it applies a slightly greater load on the higher voltage cells. The voltage at S1 is 958mV and at S2 it is 978mV so this much imbalance (1/24 or about 4%) shows up as a 20 mV difference out of 1.000 V or 2%, which is easily discerned with the 10 bit ADC.

I may try and make a prototype for this device. I'm working on another, similar, project and I might be able to use the same basic hardware for this design. If it seems worthwhile I might make a PCB which will greatly simplify assembly. I made the other special VRLY for another forum member for $100 + $10 shipping, and I'd want about the same for this design, if you want it soon. Otherwise it may have to wait until I get "a round tuit".


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## lnpurnell (Sep 27, 2012)

PStechPaul said:


> Here is a simulation of the center tap balance monitor using 7 and 8 3V cells. The voltage at the two sense points S1 and S2 is 1.000V because of the chosen values for the resistors corresponding to the two half-pack voltages. The current out of each battery is exactly 1.000 mA.
> 
> 
> 
> ...


Hi Paul,

That sounds good to me, I am still not 100% on the centre top balance in the voltage sensor relay (I would like to keep it as simple as possible to begin with) but if you can make a prototype PCB board that would be the best. 

I am not in a rush with it (I am not fully starting on my project until the new year) but I would be happy to cover those costs and when you can get ' a round tuit' My strengths have never been in electronic PCB design unfortunately.

Let me know if this is OK.

Leigh


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## PStechPaul (May 1, 2012)

That sounds like a reasonable plan. I'll make a prototype in a few weeks or so and see how it flies. If others are interested, that would help give me more incentive to "git 'er done". One thing I need to know is what sort of package this should be in, so I know what size to make the board. 

I can fit it to the same size as the original VRLY, 1.75" x 1.20", which fits in an octal relay can. It looks like this:


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## lnpurnell (Sep 27, 2012)

PStechPaul said:


> That sounds like a reasonable plan. I'll make a prototype in a few weeks or so and see how it flies. If others are interested, that would help give me more incentive to "git 'er done". One thing I need to know is what sort of package this should be in, so I know what size to make the board.
> 
> I can fit it to the same size as the original VRLY, 1.75" x 1.20", which fits in an octal relay can. It looks like this:


Hi Paul,

Whatever format is the easiest will work for me, the only requirement is that it is relativly compact and the push button for the manual NO relay will be easily accessable.

An LCD screen to read out the voltage might be a nice option for the future but I think we should leave it off until the main requirements work well.

I will PM you my email so we can keep in touch,

Thanks,

Leigh.


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