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Should you install a Battery Management System to protect those expensive Lithium batteries?

LiFePO4 battery cells cost money.


The 45 cell LiFePO4 battery pack in my electric MR2 costs $10,500.

A Battery Management System (BMS) can easily cost another $1200.

If you are considering a set of Lithium batteries for your conversion, should you invest in a BMS to protect your batteries or not?

I chose not to, and it has served me well so far after almost 2 years and 20k miles. I will show you how I protect my batteries.


What is a BMS?

LiFePO4 cellsA battery management system provides some basic functions to protect the battery cells. They are:

  • While charging, provides high voltage cut off to the charger when individual cells reach critically high voltages.
  • At the end of charging, it balances the cell voltages by shunting the current on cells that are fully charged, while allowing the less full ones to continually charge.
  • While discharging, it provides low voltage reduction of throttle when individual cells reach critically low voltages.
  • Provides constant monitoring of each individual cell.


Why did I choose not to install BMS?

  1. Added complexity to install.
  2. Reliability: A couple of friends have told me stories of the BMS not being very reliable despite the cost.
  3. Cost.
  4. Physical constraints: I’ve installed batteries in the tunnel where the fuel tank used to be. The cells barely fit into this tunnel, thus, adding extra height from a BMS, which did not work for my configuration given the BMS’s that were available then.


My solution

Battery Management SolutionI chose to do frequent manual monitoring combined with bottom balancing when needed. It takes me less than 2 minutes to measure the cell voltages of all 45 cells with a simple monitoring device. Step-by-step instructions to build one are included below.




Advantages of a simple monitoring system

  • Simple
  • One less component (the BMS itself) to worry about failing.
  • Costs less than $250 (not including tools) instead of $1200.
  • Doesn’t add to the height of the battery footprint when space is constrained.


Downsides of not having a BMS

  • I must be careful not to overcharge the pack since there is no high voltage cut off while charging. To mitigate this, I just set the charger to a conservative shutoff voltage of 153V. This means that the cells should have an average of 3.4 when fully charged. Also, take a look at the voltage curve below. Between 0 to 5% discharged capacity (95-100% full), the cell voltage of a LiFePO4 spikes from 3.3 to 3.6 volts. This property can help prevent cell damage by helping the pack reach a full voltage of 153V quicker to shut off the charger, as long as the pack is reasonably balanced. Having a lower shutoff voltage means a bit less usable range, but for my daily commute of roughly 50 miles round trip for work, the 70 mile range is more than enough.

Lithium cell voltage curves

  • No low voltage protection during driving, if I ever have to drive more than my normal range of more than 70 miles freeway, I risk damaging cells by drawing too much on the low cells so I must plan ahead by locating charging stations near where I am going. (My recent range experiment was really nerve wrecking). Having some sort of “fuel gauge” to know how much energy you have used is very useful and I highly recommend the EV display 2 from CleanPowerAuto LLC.  It uses a hall effect sensor to count the energy used as a way to measure the amount of “fuel” left.
  • No auto balancing of pack. I must periodically keep an eye on the cell pack voltages across each cell to make sure they are in balance. As I mentioned, I will show you how to make a simple quick connect device made with cell log 8s to quickly measure each individual cell voltage on a regular basis. If an imbalance is detected, I use a DC lithium battery charger used by remote control hobby enthusiasts to charge the low cells.
  • No constant monitoring of each cell.  However, having a device to quickly measure the cell voltages makes monitor easy and convenient. Again, after 2 years on the road and driving 50-60 miles per day, with only monthly measurements, the batteries have not been out of balance too much.


For those contemplating BMS solutions for your conversions, I hope this tutorial is helpful to you.   For those of you that have invested in a BMS, please comment below on how your experience has been.  Or if you use something similar to my solution here, please offer suggestions or ideas of improvement. Thanks!


How to build your own inexpensive monitoring system using cell log 8s

SAFETY NOTE: Follow these directions at your own risk; take care to prevent shorting of battery terminals.  There are high voltages involved, so be smart and be careful!

Also, you should measure your pack voltages individually first to see if you will need to bottom balance first. Here is a tutorial on bottom balancing.


Step by Step

Step 1:  Order components and gather tools.

  • 6 Cell log 8s: each can measure 8 cells. I have 45 cells, so I need 6. $15 each x 6 = $90
  • 6 Cell log 8 9-wire connector: about $2 each x 6 = $12 (just search for “cell log 8 wires” on Ebay)
  • 3 DPDT switches from radio shack. $3 each x 3 = $9
  • 1 Styrofoam block: $3
  • 1 wooden Box using 3/8” plywood (size is 11.5 in. x 5.5 in. x 2.5 in., assuming 6 cell logs) : $10
  • 6 sets Molex 9-pin quick disconnects male and female (Radio Shack): one for each cell log 8. $7/set = $42.
  • 45 1/2 amp fuse and fuse holder: $15 for a lot of 100 fuses, $2.99 for a lot of 10 fuse holders. (on Ebay: search for “AGC fuser holder” and “½ amp AGC fuse”)
  • 1/8” clear plexiglass cut to size (11.5 in. x 5.5 in.): $5 from any hardware or plastics store like Tap plastics


You will also need these following items even if you decide to get a BMS.

  • 22 gauge wire: make sure you get enough; you will need to run a line from each of your cells to the central measuring point. ~$30
  • 45 ring terminals: $6 for a lot of 100 (on ebay search for “high temperature ring terminal 5/16 stud” )
  • 3/4” Wire loom tubing to protect the monitor wires: any automotive store. $10
  • Heat shrink tubing to insulate soldered connections: (on ebay search for “heat shrink tubing”) $5



  • Crimper for 22gauge wire and terminals: Here is an inexpensive ($75) and very flexible ratcheting crimper that I got from amazon (and love!), the heads are interchangeable and crimps different terminal types from 8 to 22 gauge in size. Tool Aid modular crimper (highly recommended)
  • Soldering iron: Find one with a fine tip $15-20.
  • Heating gun, $25. A hair dryer will work, but will take a bit longer because it is not as hot.  Picture of heat gun
  • Multimeter: Get a quality one as you will use this to calibrate the cell log 8′s. I’ve had bad experiences with very inaccurate voltage readings while the batteries were charging from using a cheap $10 one. I use a 22-812 digital multimeter from Radio Shack ($70).RadioShack22-812 PC Interface 46-Range Digital Multimeter
  • Battery charger: I have the GT Power A6-40 DC charger ($42):GT Power A6-40 charger
  • Battery charger power supply: Yes, you have to buy this separately. I wish they were just included with the charger: A30/9205 25A 14V DC 350 watt power supply ($39).

A30 power supply for DC chargers

  • Wire stripper for 22 gauge wires: ($20)



Step 2: Run the wires and properly label them and the batteries.

Run the wires from each of the battery terminals to a central location where you will be taking your voltage readings. I’ve seen some folks put this in the trunk so that it is out of sight.  I chose to run it to the inside the cabin on the passenger side.

SAFETY Note: There is high voltage potential from contacting the quick disconnect pins, it is important to cover these connectors when not in use. Also, make sure you put the female end of the connector on the battery side (not the cell log side) to minimize shorting risk. It is also important that you protect the wires with ½ amp fuses close to the battery terminal so that in case there is a short, the fuse will be there as a protection mechanism.

Protect the wires with wire loom tubing through holes in the body of the car.  Give yourself plenty of extra wire to work with one each end. You will need to solder or crimp the terminals and quick disconnect pins to the wires on each end. Label each wire on both ends with a cell log number and a wire position number. For example, Cell log 1 and position 1 would be 1-1. Label the battery terminals like this (click on image to enlarge):

Label batteries for battery monitor system.Starting from the most negative battery first, label the negative terminal 1-1, label the next battery’s negative terminal 1-2, and the next battery’s negative terminal 1-3. At the 9th battery, mark the terminal 1-9 as well as 2-1. This is the start of the 2nd cell log.




Step 3: Battery side connections

For each wire on the battery side, solder the fuse holder and then crimp a ring terminal to the other end of the fuse holder wire. Like so:

Ring terminal to fuse holder to wire


Step 4: Measuring side connections

On the other side of the wires (at the central measuring point inside the cabin or the trunk), crimp the female quick-disconnect pins to each of the wires. When complete, snap the pins into the female quick-disconnect plug in order wire 1-1 through 1-9 goes to quick disconnect #1 holes 1-9 (If you look carefully, you should be able to see that the holes are individually labeled 1 through 9; match wire 1-1 to hole 1, 1-2 to hole 2, etc…

female molexlabeled molex

Step 5: Connect the cell log wires to the male quick disconnects

Crimp a male pin to each of the 9 cell log wires and snap the pins into the quick disconnect plug by matching the wire positions to the hole numbers. For example, position 1 wire goes to hole #1.

molex plug with male pins


Step 6: Add the DPDT switch

Cut the cell log 8 wire at position 1 for each of the cell logs and solder about 1 ft. of 22 gauge wire to each end of the cut wire. Connect the two 22 gauge wire ends to the DPDT switch. One DPDT switch can control power to 2 cell logs. In diagram B below, Connect the 22gauge wires from pin1 of cell log 1 to A and B, and connect the wires from cell log 2 to D and E. Do the same for cell logs 3 and 4 then 5 and 6. This switch will turn the cell log on and off.

cell log 8 connected to DPDT switch

Diagram A



DPDT diagram

Diagram B.


Step 7: Build container box

Build a wooden box out of 3/8” plywood which will be large enough to hold the number of cell logs that you will use (again mine is 11.5 ft. x 5.5 ft x 2.5 fit) as shown. Secure the cell logs using foam by cutting the foam so that it fits inside the box, then, cut out place holders for each of the cell log 8s. Drill necessary holes for the cell log 8 wires and the DPST switches.

Cell log 8 box


Step 8: Secure cell log wires to the cell logs

Secure the cell log wires to the cell logs with some electrical tape so it does not come loose when unplugging the quick disconnects later after measurement.

celllog 8 with wires



Step 9: Connect the monitoring wires to the battery terminals

Connect the ring terminals labeled 1-1, 1-2, and so on to the battery terminals by matching the labels you made in Step 2. Be sure to install ring terminals on top of the copper bus bar to prevent high current from traveling through the monitor wire.

Monitoring ring terminal on top of bus bar

Monitoring ring terminal on top of bus bar



Step 10: Record cell voltages with multimeter

Measure and record the individual voltages on the female quick disconnect side with a multimeter to ensure proper connections. Redo any problem Connections. The recorded measurements will be used to calibrate the cell logs later.

Record all cell voltages using multimeter

Step 11: Calibrate cell logs

Connect each cell log 8 using the quick disconnects and switch them all on. Calibrate each cell voltage reading using the values recorded in Step 10 per calibration instruction found in the cell log 8 manual.  You are done!  Now you can use the monitor to quickly get an overall feel for how the cells compare to each other in less than two minutes.




Important Note:

DO NOT leave the cell logs connected to your batteries even if switched off. The cell logs have parasitic amp draws ranging from 0.02mA to 0.760mA across each pin even when they are turned off. These amp draws may not seem like much, but if you leave them connected, over time, they will contribute to pack imbalance. Here are some data on the amp draws from each pin of a couple of cell logs (switched ON and switched OFF) that I collected:

PIN cell log 1, OFF, amp draw (mA) cell log 2, OFF, amp draw (mA)
9 0.57 0.76
8 0.02 0.02
7 0.33 0.35
6 0.04 0.02
5 0.12 0.15
4 0.20 0.18
3 0.27 0.35
2 0.30 0.32


PIN cell log 1, ON, amp draw (mA) cell log 2, ON, amp draw (mA)
9 0.81 0.78
8 0.762 0.74
7 14.82 15.60
6 0.02 0.02
4 0.02 0.08
3 0.04 0.04
2 0.02 0.03
1 15.84 16.64

Figure below is a diagram of 1 of the 2 cell log 8′s amp draws that I measured.

Cell log 8 amp draws ON and OFF



Final thought: an argument for getting a BMS if you can afford it in your conversion budget:

It is possible to get 3,000 cycles on these LiFePO4 batteries provide I stay within 70% depth of charge. That is more than 10 years of use or 150,000 miles (50 miles daily commute x 3000 cycles) on these batteries . This could mean a savings of at least $18,750 in gas money (assuming gas prices stays at $4/gallon and a gas car MPG of 32). My pack was $10,500, so I am still $8250 ahead. If I spend another $1200 on a BMS that will help me prolong the life of these cells, I am still ahead. I may still consider installing a BMS at some point in the future. But for now, my cell log 8 monitoring system is working well.



Related Posts

Posted in Batteries, BMS, Cell log 8, Charger, Instrumentation, MR2EV

11 Responses to “Should you install a Battery Management System to protect those expensive Lithium batteries?”

  • Ken says:

    I posted this on DIYElectric Car. There seems to be a heated debate on whether BMS are worth the money or not and the poll seems to suggest that the split between BMS and no BMS is pretty evenly split. Someone on the forum asked me about the extent of my balancing so far and here’s my answer:

    Re: Battery Balancing / BMS Holy War

    Originally Posted by GerhardRP

    Can you give details of the balancing you have done?

    My response:
    Looking back in my battery logs, monthly, I’d measure the cell voltages of each cell after driving the car to about 30% charge remaining. The average cell voltages at this state of charge is typically 3.22 +/- 0.02 volts. If they are significantly lower than this, I will use my GT Power battery charger and individually charge the cells until their voltages are in par with the rest of the cells. I do this in increments so I don’t have to watch the charger or risk forgetting about the charger. I’d preset the amount of energy to add to the battery, when it is done, I’ll measure the voltage again until it reaches voltage. In my experience, it hasn’t been more than 2-3 AH that I would add to the low cells to get them balanced.

    I’ve had to add charge to low cells 2 times in the almost 2 years that I’ve been driving, the first time was on 3 batteries and the 2nd time on 2 batteries. It has not been bad at all.

  • Ken says:

    I forgot to mention an idea to make things easier. For those of you that don’t like to solder or think it is too time consuming to solder all of the leads in step 3 above, You could use these connectors.

    A friend that works at Tesla recommended them and I gave them a try for something else and they are amazing. Just cut the wires, strip, insert into the levers and engage the levers and you are done! It creates a very solid connection. If I do another conversion, I definitely will use these to save time.

  • Mark Brems says:

    Hey Ken,

    There is one issue you describe above that is not a problem with a non-distributed BMS, like the Orion that I am using. Any distributed BMS will usually require the attachment of a small monitoring/shunting module between the + and – terminals of each cell, which adds to the vertical profile of your cells. This is a big issue when squeezing cells into spaces for which they were not designed. Also, it is not very helpful when those modules have status lights which need to be visible. A non-distributed BMS centralizes all BMS functions in a main unit that can be placed anywhere in the car, with the only requirement that a single cell tap wire run from each cell’s positive terminal to the BMS unit. These are small gauge wires that take up less space than the busbar connections. All cell conditions are visible on a central display from within the cabin. Very convenient, and without the worry.


    • Ken says:


      The centralized BMS would be a good way to avoid the profile problem. Have you had problems with less accurate voltage readings from your centralized BMS? I’ve heard that they are not as accurate as distributed ones and and began to shunt charge current too early. Take a look at the very end of this post:

  • Joey says:

    I think you did a nice job of summerizing the key aspects of the BMS debate. I would say monitoring is a must (either manual or automatically seems to be fine). Your comment that you must be careful with high and low voltage cutoff is the key. And another benefit of being careful: there is a growing consensus that cells will last longer if you stay away from the extremes of full and empty state of charge. I chose to get a BMS because I want to pour through all the data as a hobby. I do not plan to use the balancing function automatically on every charge. I think that the cells should be balanced while being observed, and that they should be balanced when they are as close to a resting state as possible. The cell voltage is too unpredictable right after a charge cycle or heavy discharge cycle to get a reliable balance. I agree with the reliability comment too. That is why I want the charger to be responsible for a safe charge termination and the BMS will be a backup system. We will see how it works out as I only have 50 EV miles so far, and I don’t have the BMS installed yet.

    • Ken says:

      Hi Joey,

      Thanks for visiting my blog! And congratulations on getting your electric Porche on the road! Please consider adding your EV to the EV Club at There is a widget that you can get which displays your miles driven right on your blog.

  • Stephen says:

    Excellent cell monitoring alternative. Thank you very much for sharing all of the details including the material list. I am attempting to replicate for a 16-cell pack in a golf cart. Having trouble locating the connector for the Cell Log. Can you direct me to a link?

    Considering this alternative to the Cell Log: Hobbyking 2-8S Cell Checker with Low Voltage Alarm.

    Until I find the connector, I might install four LED RX Voltage Displays with each showing the voltage of a 4-pack.

    Any input or words of wisdom would be appreciated.

    • Ken says:

      Glad the details helps you!

      I’ve had trouble finding these connectors as well, Ebay is the best source.

      Here is a recent listing:
      Cell log 8 connector wires

      You can also search for 8S-JST-XH on ebay.

      Good luck and let me know how it turns out. Thank!


      • Stephen says:

        Ken, Thanks. After some searching, I found that sells a pair of Balance Lead Extensions. Their customer service confirmed that the Cell Log uses a JST-XH 8S connection (9-wires). I will cut off the male end and fit the ends of the bare wires with spade connections to mount on a 10-post terminal strip. The two terminal strip will be inside a 5″ x 5″ plastic project box secured to the dashboard. The connector will hang out the side of the box for connecting to the Cell Log. I will place Velcro on the back of the Cell Log to allow attaching it to the outside of the project box so that it is in full view and the alarm can be heard. This will also accommodate unplugging the Cell Log after trips. Steve

        • ken says:

          Stephen, thanks for sharing and correcting the number of wires on the connectors. I’ve fixed the post from 7s to 8s. Also, I’d love to see some pictures if you are willing to share. My email: Ken at mr2ev dot com. Thanks!

          • Stephen says:

            Sure thing. It will be the very end of December when everything is assembled. The pair of Cell Logs are on order as well as the ring terminals, fuse holders, fuses (200 mA), spade connectors, project box and terminal strips. Will obtain some split loom, Ty-Raps, and Velcro at a local hardware store. My intention is to use the Cell Logs whenever the cart is in use. That will provide full cell monitoring and alarm. Will unplug them when not in use.

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