This article is in 2 parts:
part 1:direct comparison of sealed lead acid batteries & lifepo4
part 2:BMS facts and how to charge lifepo4 without it!
This article is about batteries and my personal experience with both sealed lead acid (SLA) and lifepo4 batteries, and most importantly how to care for your lifepo4, and not make the same moronic mistakes I have made in using my lifepo4, I had to learn the hard way through personal experience, hopefully this article will save you money and heartache.
SLA or lifepo4? There is NOTHING TO THINK ABOUT HERE! Ive used both and hands down lifepo4 wins the race.
When I got into building my first E bike I ordered lifepo4, but at the time the technology was IMHO good as far as the batteries are concerned but there were problems with the battery management systems, I say the batteries were good because I still have my original 48v20ah lifepo4 pack that was bought back in august 2007 now were may 2009, my whole pack is perfect except for one cell which is now completely dead, I started to notice its demise about 2months after receiving the pack it was the weakest cell and I monitored it closely. So far Ive gone through 70 charge discharge cycles at 70% depth of discharge (DOD). The charger BMS system died after only being used approximately 10 times. I will explain in detail later in this same article how I was able to continue charging my batteries without using the BMS provided to me by the Lifepo4 Company that sold me these batteries in the first place.
Here is a story of a direct comparison of using standard sealed lead acid batteries and then switching over to Lifepo4
My friend Chris bought a pack of lifepo4 48v10ah at the same time as me, we purchased together from the same company in china in order to save on shipping, to this day his pack is perfect. And he has gone through at least 250 cycles on his pack. Soon I will get back to lifepo4, now I want to tell you about SLA.
My bike was ready to roll mid July 2007 but I was still waiting for my lifepo4s, I had ordered them back in April 2007 but the company was making all kinds of excuses until the pack was finally delivered mid august 2007. (My guess is they were having problems with the BMS)
I already had 2 SLA 12v20ah batteries from my first 24v e bike so I invested in one more battery to bring my total to 36v20ah.i was king of the road, my goal; to be able to drive 22km to get to my girlfriends house. Well the only way that was going to happen I found out quickly enough was to do an enormous amount of pedaling, cause at only 12 km of riding you could really begin to feel the voltage sag, and at 16km the low voltage cut-off (LVC ) from the controller would kick in. at 17km game over. So from that point on I had to divide the ride between pedaling and battery power to arrive home sweating bullets, no problem on my part I liked the exercise and always laughed it off.
Here is the good part, for the whole month while waiting for my lifepo4s I kept thinking what a moron I was to think that these new miracle batteries were going to change my 22km 36v20ah SLA strenuous ride (don't forget the SLAs were good for only 17km but my destination was 22km) into a 48v20ah lifepo4 easy as pie ride.
I was very suspicious about this one teensy aspect; my SLAs weighed in at 39lbs giving me a total 36v20ah, and was giving me 17km per ride, all the time running through my head,ok Im at 12 km and these batteries are sagging, yet the lifepo4 pack that I will receive hopefully this year, weighs in at only 26lbs and yet supposedly has 48v20ah, my next thought,boy what a jerk I am there is no way in hell that a pack of batteries weighing just 26lbs will take me as far as a pack that weighs 39lbs, and my next thought, just be happy that if I ever receive these batteries that maybe it will take me close to the 22km I need to travel.
Well mid august the lifepo4 batteries finally came, I measured all the cells and they all matched at 3.3v, fully charged I was at 53v total (my SLAs would be at 41v fully charged) so this was exciting but I still could not help thinking well this pack only weighs 26lbs so don't get too excited lets see how far I go on a charge.
It goes without saying 53v was really exhilarating not to mention the fact that my bike was now carrying 13lbs less batteries.
12kms into the ride hmm, I'm going upwards of 40and 50kmph and there is absolutely no sag, hey Im sag-less!
17km into the ride...holy moly, I'm still going strong18, 19,20, 22kms its full steam ahead into the driveway! I heard angels singing, halleluiah!!!
Now I grab my voltage meter, WTF I'm at 52v what is going on here??? yell out to my wife (she may as well be, we've been together 7 years) Ill see ya later, I just had to ride to see when these batteries would conk out, well driving like a lunatic I managed 32kms range in total.
I will never ever go back to SLA's, they are heavy and cumbersome, and give lousy range and an overall lousy ride.
So here are the facts: My lifepo4 pack weighs in at 26lbs and is rated at 48v20ah giving an exhilarating strong ride at 40 and 50kmph and a range of 32 kms using a 36v to72v40amp controller on 28" 700b wheels I weigh in at 160lbs clothed.
SLA's weighing in at 39lbs gives 36v20ah slow saggy ride approximately 36kmph and only 17kms range also using the 36 to 72v40amp controller on 28 700b wheels . Whats really amazing here is that at 39 pounds I'm only at 36 V 20 amp hours in order to go to 48 V I would have to add another 12 V 20ah battery bringing me up to a very heavy 52 pounds! I have no doubt in my mind that even with that extra battery I will get nowhere near the performance of my 48 V 20 amp hour Lifepo4 pack Weighing in at only 26 pounds!
(Note: I have since had to change the controller to the new 48v35amp 12fet IRFB4110) (also note that my Lifepo4 battery pack was bought in 2007 whereas today's newer packs are probably somewhat lighter, also it must be stated that depending on the company you are dealing with, they may sell Lifepo4 that are packed in foil which are even lighter as opposed to the plastic prismatic cells, the problem with the foil packs is each single cell cannot be monitored separately as they are tightly packed together and shrink wrapped all wired up to a BMS.)
BMS facts and how to charge lifepo4 without it!
As I said earlier my charger BMS died after only 10 uses. So you may be wondering what a "BMS" is, it stands for "battery management system".
Now I will unveil the mystery of what a BMS actually does, why it's important for the health and well being of your life batteries, and, here is the best part, this will be explained in "laymen terms."
No complicated electronic jargon here, this is for people that just want to know how to take care of life batteries and maintain them properly. so for anyone who wants to know about the electronic doo hickys inside a BMS you won't find it here.(note to all electronic geniuses, thank god for you guys, cause if it were not for you, the world would be a pretty boring place IMHO)
First off, if you intend on owning lifepo4 batteries you do have to learn one thing and that is how to use a voltage meter, its very easy especially if you only use it to check your lifepo4 batteries. If you do not want to learn how to use a voltage meter, then dont expect to get the optimized life expectancy of lifepo4 batteries, they should be monitored every few weeks and it only takes maybe 5 minutes of your time to save your investment.
Do you really need a BMS to charge lifepo4?
Contrary to popular belief YOU DO NOT NEED A BMS TO TAKE CARE OF LIFEPO4 BATTERIES, however if you do not use a BMS you must monitor your pack manually, the good news here if your pack is well balanced to begin with you will only need to check your pack maybe every 20 to 40 charge discharge cycles. Here is how you can do this.
Each cell fully charged should register 3.3v up to 3.65v
The problem with lifepo4 batteries is they cannot be over charged, if you overcharge them they will be damaged to the point they will die. They basically will not be able to hold a charge.
another problem lifepo4 batteries have is they cannot be over discharged ; if they go below 2.2volts well you have done a very bad thing; once it goes below 2.2v it becomes starved for more energy, and if you run it for long enough below 2.2v it simply will never be able to take a charge, when a person drowns they will be unconscious if you revive them in a minute or two chances are they will be just fine, but if that person has been unconscious for 5 minutes or more, well unfortunately chances are that person will suffer long term damage. Its the same for lifepo4.
There are three ways to make sure you don't overcharge them;
1: make sure the charger you use is matched to your pack, for example if your pack is 48 V20 amp hours you would use a charger that is rated at 48 V at perhaps five to 8 amps.
2: you use a BMS with your charger
3: you use single cell Chargers made special for lifepo4.
Here I will explain the three different ways to charge lifepo4 batteries.
There is only one way you can really insure that your battery will not overcharge or discharge: this is where a BMS really comes into play and why companies that manufacture Lifepo4 want people to use them, the battery management system monitors each cell, when a cell is fully charged it should register at approximately 3.65 V, the battery management system senses this and it will not allow that particular cell to receive any more charging from your battery charger while at the same time it continues to charge the other cells until all the cells reach 3.65 V.
The battery management system (BMS) also incorporates a low-voltage cut off ( LVC), because the battery management system is wired to all the cells in your pack it has the ability to sense each individual cell, so if in fact one cell is weaker than the others, as soon as it reaches 2.4 V it will shut the power going to your controller, saving that particular cell from being over discharged.
So now you know why the battery management system is so important. Without it you have a much greater chance of ruining your battery pack. Remember when I said earlier YOU DO NOT NEED A BMS TO TAKE CARE OF LIFEPO4 BATTERIES, you don't, provided that you're willing to monitor the pack and that you fully understand that if in fact you have one cell in your pack that is not balanced with the rest of the pack and is in fact weaker than the rest of the cells and if you solely depend on the low voltage cut-off of your controller, well because of that one weak cell over time your battery pack will in theory become very unbalanced, and in turn will dramatically decrease the life of the overall pack. The reason for this is simple; the stronger batteries in the pack will keep the low-voltage cut off from your controller cutting out the power to your motor in a timely fashion, (remember: we are talking about the low-voltage cut-off from your controller not a BMS )for example you're 48 V pack should cut out at 42 V and in normal circumstances it will, but just because it is cutting out the pack 42 V, that does not mean that all the cells in your pack have not gone below 2.4 V it simply means that the overall pack has gone down to 42 V, so some cells may be at 2.6 V while others can be at 2.2V, 1.8V, etc, well here it goes without saying that the cell at 1.8 V is very likely ruined. IF you do not understand this paragraph AND you want to operate your Lifepo4 pack without a battery management system then you must understand what is said here. IF you don't understand this you definitely need a battery management system.
When I first received this pack it came with a very complicated charger and BMS (battery management system) each cell had a set of wires that would do two things:
1: charge the batteries to 3.65v and make sure that each individual cell would be matched to the voltage of the weakest cell thus balancing the pack.
2: there was an onboard LVC (low voltage cut-off) that would stop the flow of power leading to the motor controller once it sensed that any cell fell to 2.4v.
Well all that was very nice until about 10cycles worth of charges the BMS stopped working. So now I call the battery company and they want $165.00 for a new BMS and take it or leave it.
So now I'm not a happy camper and because they were not reassuring that the same thing wouldn't happen with a new BMS I set out to find a more reliable one. Well back in 2007-2008 there were almost no options available and I thought that the batteries were going to end up being useless. So through some research and asking for help on this great forum endless-sphere someone recommended to use a 12v smart charger and charge in series 4cells at a time, at that time I thought this guy is nuts if he thinks I'm going to experiment around with my $700 pack of batteries only to ruin them with a regular charger. Well after checking around it turns out he was not so nuts, maybe crazy, but not nuts. So I took the plunge and charged up 4cells with my 12v smart charger and to my surprise they charged up very nicely and they stayed balanced. So for the next few weeks I charged them like that 4 cells at a time and then invested in a Soneil 48v3.5amp charger. Turns out that this charger is perfectly suited to lifepo4 chemistry, I now charge my whole pack of 16cells which is 48v20ah pack lifepo4 since April of 2008 and to this day (sept 2009) the pack is still perfect and has gone through at least 90 charge discharge cycles.
To be safe I ordered 4 X lifepo4 3.65v2amp single cell chargers and for the first time last week (around june5) I charged my pack using the single cell chargers and they all took the same amount of time to charge up to full after a 22km ride (approximately 3hrs each cell) which means that the overall pack is still perfectly balanced and I could of just kept going with the Soneil charger.
Now unfortunately there is another problem that has to be addressed with owning a lifepo4 pack, UNDER NO CIRCUMSTANCES DO YOU LET ANY ONE CELL GO BELOW 2.2V. Let me repeat that UNDER NO CIRCUMSTANCES DO YOU LET ANY ONE CELL GO BELOW 2.2V. If a cell goes below 2.2v there is a very good chance you have permanently damaged that cell.
How to protect your lifepo4 from going below 2.2v without a LVC monitoring your whole pack
Here you have to know how far you can go on your pack assuming that all cells are healthy and paying close attention if you have to go to an 80% depth of discharge (DOD).
Also when taking the voltages from all your cells pay attention to which cell is the weakest in the pack, you can then wire that particular cell and have those 2 wires easily accessible in order to take a reading, as soon as you see that you're hitting around 2.4v to 2.6v then its time to start pedaling and immediately re-charge.
notice in the pic the 2 wires coming from my weakest cell in the pack
On my 48v20ah pack I know that I can go up to 32km maybe even 35km,but because I have no LVC (for now, cause one day I will install one as they are becoming more and more accessible) I will never go beyond 25km to be safe. If you notice in the pic below you can see the test wires coming from my weakest cell. So the trick here is to know the parameters of your pack.
When I first got my lifepo4 pack I foolishly brought down the pack to the LVC of 2.4v and then waited for the volts to go back up and then continued riding. Well I paid the price in finding out that one of my cells was predominately weaker than the rest and sure enough it eventually died, so now instead of 16cells I am now using 15cells (Im waiting for a new cell to arrive and of course this company is taking forever to deliver)
The good news is the other 15 cells are still running great and because I monitor my pack properly and take care in its usage I believe it will serve me for years to come. My friend has a pack from the same company and has gone through at least 3 times as many charge discharge cycles at 80% depth of discharge (DOD) and his pack is 48v10ah and he too has no BMS. Since first writing this I'm happy to say I got my new cell and I'm back to 16.UPDATE it is now July 2011 im still using the very same 48v20ah pack and it is still perfect,,,when on the rare occasion i go for an extended ride at about 25kms the pack starts to sag a bit so now instead of 32km range at full charge I'm down to 25kms range at full charge,,,not bad for 5 yrs usage,,,,,and its still going strong.keep in mind i drive fast,full throttle all the time almost no pedal,speeds between 38 to 45kmph easily topping out at 50kmph on straights a mile and more.as for my friends pack his stopped working completely about may 2011,,,he lives in Toronto and it was his sole means of transportation,,,its no exaggeration that he used this pack 200 to 250 times a year for the course of the five years running the pack to 80% DOD,,,,his pack was only 48v10AH so he only had apx 15 to 18kms range till his pack is depleted,,,and all this without a BMS!
Last edited by maxwell65
on Sun Jul 31, 2011 11:21 am, edited 2 times in total.