Protection of Lipo in parallel

[salamandra3331]

Hello everyone

I work with LIPO for the first time and really afraid that things can blow up if I abuse batteries. In short, I have to power few motors and servos connected to four LIPO 3S1P 11.1V 5500mA batteries through ECSs. My concern is in overdischarge and parallel connection itself. I couldnt find any suitable small IC to monitor each cell inside the batteries  and so far I stopped on the idea to use some microcontroller for that purpose.

Basically, my idea is to read up voltage values from each cell inside battery packs and shut connection to load down with mosfets when cutoff is reached(3.5V). Something like multiplexer + MCU could do the job with minimum pins required.

To protect batteries from impact on each other I thought to put schottky in series but I would really prefer to avoid extra power losses. I think PMOS wouldnt do the job and fuse probably is not best solution cause of maintaince.

Overall Im not so confident that it will work out as I expect and that this solution is best. Maybe somebody has more experience and can suggest some better ideas.
Thanks in advance

[BravoV]

To protect batteries from impact on each other I thought to put schottky in series but I would really prefer to avoid extra power losses.

I'm NOT an expert though, and not sure if its suitable, have you look into TI SM74611 smart by pass diode ?

Although there is a catch, it will not conduct all the time like ordinary diode, as the internal circuit needs to recharge the cap to turn on the mosfets, just check it's datasheet for the detail.

[CraigHB]

The only issue with parallel LiPo batteries is that large equalization currents can occur upon connection if there's a big mismatch in state of charge.  LiPos are typically very high drain so it's not a problem for output, but there's a factor of around ten between output current tolerance and input current tolerance.  So if they differ enough in state of charge it's possible to exceed input current tolerance on one of the batteries. 

Once batteries are connected in parallel and equalized, they behave as a single larger battery.  For safety considerations your option is to either permanently connect them or simply use a single larger battery with equivalent capacity. 

You need to protect LiPos (or any Li-Ion battery for that matter) from over-discharge and over-current.  Typically off-the-shelf ESCs have over discharge protection where minimal voltage levels can be programmed through software.  Some even have over-current protection, though it's not typical. 

It's a bit involved adding over-current protection with high current devices.  It's done with a current sensing circuit and an input power MOSFET that shuts down the circuit when excessive input current is detected.  That's not a particularly difficult circuit to design, but it does call for a ucontroller.  A low pin count 8 bit device can do the job with a few lines of code.

For lower currents Li-Ion protection boards off the shelf can do the job, but you don't find them commonly with high enough ratings for electric motors doing larger mounts of work.

[salamandra3331]

The only issue with parallel LiPo batteries is that large equalization currents can occur upon connection if there's a big mismatch in state of charge.  LiPos are typically very high drain so it's not a problem for output, but there's a factor of around ten between output current tolerance and input current tolerance.  So if they differ enough in state of charge it's possible to exceed input current tolerance on one of the batteries. 

Yes. Exactly for that reason I would like to have additional protection to increase reliability rate of my product. It is crutial in the design. I would love to eliminate even slight chance that current will go "into batteries".

You need to protect LiPos (or any Li-Ion battery for that matter) from over-discharge and over-current.  Typically off-the-shelf ESCs have over discharge protection where minimal voltage levels can be programmed through software.  Some even have over-current protection, though it's not typical. 

Yes. My ESCs have discharge cut off voltage but only total battery pack output voltage is monitored for that reason. What I mean is that if at least one cell inside any pack gets overdischarged - it will damage battery pack. I would like to be sure that it iwll never happen. I have 4 battery packs in parallel. Each pack has inside 3 cells in series. And yes.. I know that we can assume that if batteries were charged until their maximum with balanced charging. and all of them are the same (capacity brand chemestry etc) then there will be no damage caused. However, each battery has its own internal resistance and different discharging slope. You never know how it will act if it wasnt properly tested. For these cases would be perfect to have additional protection

[salamandra3331]

To protect batteries from impact on each other I thought to put schottky in series but I would really prefer to avoid extra power losses.

I'm NOT an expert though, and not sure if its suitable, have you look into TI SM74611 smart by pass diode ?

Although there is a catch, it will not conduct all the time like ordinary diode, as the internal circuit needs to recharge the cap to turn on the mosfets, just check it's datasheet for the detail.

Wow.That is some new stuff for me. Thank you for advice. I will consider it as possible solution and surely look at datasheets. Seems like these guys are better than schottky in this applications

[CraigHB]

I would like to have additional protection to increase reliability rate of my product. It is crutial in the design. I would love to eliminate even slight chance that current will go "into batteries".

Is there some reason you absolutely must use parallel packs?  When it comes to LiPos most suppliers can provide a pretty wide range of options between capacity and C ratings.  Allowing user replaceable parallel packs adds a lot of complexity to the design only to protect against the user installing packs with a big charge mismatch.

To protect against over-discharge of a particular cell or group of parallel cells you would need a connection to the battery that includes taps between the cells.  Any fault either voltage or current can be monitored by an MCU controlling an input power cutoff.  Of course it needs to be micro-power since you wouldn't want the monitoring circuit to continue draining the battery any appreciable amount. 

A passive circuit using diodes is probably not going to do the job since they have big power losses at higher currents.  It may be possible to use active diodes.  They're available off the shelf and use a MOSFET with other electronics on a single chip to greatly reduce power loss for diode applications.  However they're limited in the amount of current they can handle.  If you're going to get into any kind of active electronics might as well just go with an MCU based design.  That would probably be easiest.

I've actually done a voltage monitor for a single cell LiPo powered device that uses only a voltage detector to protect against over discharge.  On that device over-current protection is handled by an MCU.  The voltage detector simply drives the enable pin on the device's regulator which shuts down the draw to only a few hundred nA when battery voltage falls below the threshold.  It would be possible to drive an input MOSFET with a circuit like that.  Might be something to consider.

[suicidaleggroll]

Yes. My ESCs have discharge cut off voltage but only total battery pack output voltage is monitored for that reason. What I mean is that if at least one cell inside any pack gets overdischarged - it will damage battery pack. I would like to be sure that it iwll never happen. I have 4 battery packs in parallel. Each pack has inside 3 cells in series. And yes.. I know that we can assume that if batteries were charged until their maximum with balanced charging. and all of them are the same (capacity brand chemestry etc) then there will be no damage caused. However, each battery has its own internal resistance and different discharging slope. You never know how it will act if it wasnt properly tested. For these cases would be perfect to have additional protection

That's not a very common way to connect things.  If each cell is connected in series, and then you connect the packs in parallel, every pack will need to be individually protected.  If instead you connect each set of 4 cells in parallel, and then connect those parallel sets in series to get your 3S, you only have to balance/monitor the final pack.  In other words, 3S4P (what you're describing) will need 4 balance/monitor circuits, each one testing for and correcting imbalances in each of the 3 series cells contained within that pack.  MUCH more common is 4P3S, which would only need 1 balance/monitor circuit to test and correct imbalances between the three paralleled sets that make it up (the individual cells inside each parallel set will keep each other balanced, you only need to be careful when first connecting them in parallel).

[salamandra3331]

Is there some reason you absolutely must use parallel packs?  When it comes to LiPos most suppliers can provide a pretty wide range of options between capacity and C ratings.  Allowing user replaceable parallel packs adds a lot of complexity to the design only to protect against the user installing packs with a big charge mismatch.

Yes, there were a lot of consideration taken. I was limited by some requirements.

To protect against over-discharge of a particular cell or group of parallel cells you would need a connection to the battery that includes taps between the cells.  Any fault either voltage or current can be monitored by an MCU controlling an input power cutoff.  Of course it needs to be micro-power since you wouldn't want the monitoring circuit to continue draining the battery any appreciable amount. 

Well, I have batteries with balance. I can easily monitor voltages between cells.

Btw, I found this Texas Instruments BQ40Z50RSMT-R1 IC for battery protection. If I understood it right, I can use it for battery protection during discharge mode.

[salamandra3331]

Yes. My ESCs have discharge cut off voltage but only total battery pack output voltage is monitored for that reason. What I mean is that if at least one cell inside any pack gets overdischarged - it will damage battery pack. I would like to be sure that it iwll never happen. I have 4 battery packs in parallel. Each pack has inside 3 cells in series. And yes.. I know that we can assume that if batteries were charged until their maximum with balanced charging. and all of them are the same (capacity brand chemestry etc) then there will be no damage caused. However, each battery has its own internal resistance and different discharging slope. You never know how it will act if it wasnt properly tested. For these cases would be perfect to have additional protection

That's not a very common way to connect things.  If each cell is connected in series, and then you connect the packs in parallel, every pack will need to be individually protected.  If instead you connect each set of 4 cells in parallel, and then connect those parallel sets in series to get your 3S, you only have to balance/monitor the final pack.  In other words, 3S4P (what you're describing) will need 4 balance/monitor circuits, each one testing for and correcting imbalances in each of the 3 series cells contained within that pack.  MUCH more common is 4P3S, which would only need 1 balance/monitor circuit to test and correct imbalances between the three paralleled sets that make it up (the individual cells inside each parallel set will keep each other balanced, you only need to be careful when first connecting them in parallel).

You have a good point. However, it was the requirement for me to have 3s packs in parallel.

[anishkgt]

The cells should match. Meaning each cell in parallel pack should of the same voltage. Using a BMS would be ideal and takes for over discharge and charge. I've done several packs with 18650 and I use a BMS rated for the discharge rate of the load. Checkout eBay for BMS. BMS takes care of each group in a parallel pack.


Sent from my iPhone using Tapatalk

[salamandra3331]

The cells should match. Meaning each cell in parallel pack should of the same voltage. Using a BMS would be ideal and takes for over discharge and charge. I've done several packs with 18650 and I use a BMS rated for the discharge rate of the load. Checkout eBay for BMS. BMS takes care of each group in a parallel pack.


Sent from my iPhone using Tapatalk

Hello
Do you use IC for BMS? If yes, could you specify the model please so I could have a look Thx

[anishkgt]

I use no extra hardware just the BMS for the rated load current. Something like these http://www.ebay.com/itm/3S-20A-Li-ion-Lithium-Battery-18650-Charger-PCB-BMS-Protection-Board-12-6V-Cell-/332061214304?hash=item4d50640660:g:UBYAAOSw5cNYT2r-