{"id":4102,"date":"2015-07-13T21:30:04","date_gmt":"2015-07-14T04:30:04","guid":{"rendered":"http:\/\/www.corbinstreehouse.com\/blog\/?p=4102"},"modified":"2018-11-03T13:36:21","modified_gmt":"2018-11-03T20:36:21","slug":"plug-bug-you-are-only-as-strong-as-your-weakest-cell","status":"publish","type":"post","link":"https:\/\/www.corbinstreehouse.com\/blog\/2015\/07\/plug-bug-you-are-only-as-strong-as-your-weakest-cell\/","title":{"rendered":"Plug Bug: You are only as strong as your weakest cell"},"content":{"rendered":"

\n

I had known for quite some time that I had a weak cell in my battery pack. My electric bug\u2019s battery consists of 48 lithium iron phosphate cells. I use the eLithion battery management<\/a> system with a custom display I fabricated along with some custom ardiuno code<\/a> to show cell info from the CAN bus. I would routinely see one cell (number 9) sag heavily under load.\u00a0<\/p>\n

One day I left work work (a 20 mile drive) with my pack at about 50% charged. At about five miles from work my low battery warning buzzer went off telling me that I had one cell that was fully discharged. That was my bad cell number 9, and I was only at 30% SOC once I arrived at work:<\/p>\n

\"IMG<\/p>\n

The cell was down to 2.07v, which is well below the nominal 3.20 volts, and the \u201cfully discharged\u201d value of 2.7 volts. I had ignored the low voltage buzzer, and continued to drive car, taking more energy out and over-discharging it. I knew this might destroy the cell, but I was okay with that as I had a spare cell at home.\u00a0<\/p>\n

Once I got to work, the BMS wouldn\u2019t let it charge! It thought the under voltage condition was an error where it shouldn\u2019t let any energy in (which is strange). Luckily, I could easily unplug the BMS connection to my charger, and let it charge for a bit:<\/p>\n

\u00a0\"IMG<\/p>\n

This pulled the voltage up to 2.70 volts, and let it normally charge:<\/p>\n

\"IMG<\/p>\n

At this point, I knew that 30% state of charge (SOC) was my real world \u201cbattery empty\u201d point. This one particular cell has lost 30% capacity in the 4+ years and 39,000 miles that I have been driven the car. I know the other cells aren\u2019t as bad, as they don\u2019t sag under voltage load, but this one bad cell is the limiting factor of my pack. If I continued to discharge it below 30%, it would cause the cell to die. That in itself wouldn\u2019t be that bad, but I heard dead cells can sometimes reverse the voltage and suck energy out of their neighbors. I would also have to bypass it in my BMS, as it wouldn\u2019t let the car drive with a dead cell.<\/p>\n

I continued to drive the car for a month or so, and just didn\u2019t let it get that low again, and it worked fine. But I really wanted that extra bit of range and capacity \u201cjust in case\u201d. Plus, I had two spare good cells for this very reason.<\/p>\n

Last weekend I finally did the work to replace the cell. I had previously used a volt meter to find out which cell it was (the numbering doesn\u2019t seem to be related to anything I can figure out). It was a cell in the front trunk of my car, so I pulled out the module for it:<\/p>\n

\"IMG<\/p>\n

I snapped the green strapping wires apart, replaced the bad cell, and then strapped the module back together.<\/p>\n

This time, I used my engine hoist to pull the module out and put it back in:<\/p>\n

\"IMG<\/p>\n

It was so much easier than trying to pull it out by hand (which I had done when I originally installed the modules!).\u00a0<\/p>\n

The bug now has a bit over 39,100 miles. The transmission is leaking a bit of oil from the main shaft seal; I\u2019m afraid it will eventually fail again.<\/p>\n

Now, I need to do another capacity test with the pack and see how the other cells are doing. 30% loss over roughly 4.5 years isn\u2019t that great\u2026the cells forecast an expected 20% loss after 10 years of use when discharging to more than 20% SOC. I always did this, but I also always charged to 100% SOC (indicated by roughly 3.6 volts per cell). I think always charging to 100% has reduced the capacity. If I were to do the project again, I would write some smarter charger controlling logic to limit max SOC to 90 to 95% for most charging sessions, and only charge to 100% on the days I knew I would really need it, or would use the energy right away. Sitting at a high SOC for a long time isn\u2019t good for the cells.<\/p>\n

\u00a0<\/p>\n

\u00a0<\/p>\n

\u00a0<\/p>\n","protected":false},"excerpt":{"rendered":"

I had known for quite some time that I had a weak cell in my battery pack. My electric bug\u2019s battery consists of 48 lithium iron phosphate cells. I use the eLithion battery management system… [read more]<\/a><\/p>\n","protected":false},"author":1,"featured_media":4097,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[34],"tags":[],"class_list":["post-4102","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-electric-bug"],"jetpack_featured_media_url":"https:\/\/www.corbinstreehouse.com\/blog\/wp-content\/uploads\/2015\/07\/IMG_3918.jpg","jetpack_shortlink":"https:\/\/wp.me\/p8zn47-14a","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/posts\/4102","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/comments?post=4102"}],"version-history":[{"count":1,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/posts\/4102\/revisions"}],"predecessor-version":[{"id":4103,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/posts\/4102\/revisions\/4103"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/media\/4097"}],"wp:attachment":[{"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/media?parent=4102"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/categories?post=4102"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.corbinstreehouse.com\/blog\/wp-json\/wp\/v2\/tags?post=4102"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}