Truly Maintenance Free Low Ownership Cost Project
My ride on mower is a low quality product and I was fed up with replacing the lead acid battery and fighting corrosion on the battery terminals.
No longer. It is entirely feasible to change that lead acid battery out for a LiFePO4 battery and a unique Battery Management Systems (BMS) as described below.
I expect the LiFePO4 battery to last at least 2000 starts which works out to 38 years if I start it every week of the year.
The Problem
My mower is powered by a Briggs and Stratton engine. The charging circuit is very simple, a dynamo and a diode. So the unregulated dynamo pushes several amps into the battery regardless of the state of charge. In practice this overcharges the battery requiring regular topping up with distilled water and shortened battery life.
The Options
- Keep replacing the battery with dubious quality cheap replacements
- Replace with a high cost big brand name battery and hope it lasts longer.
- Replace with a LiFePO4 battery and BMS and never have to touch it again.
Given overcharging is the norm with my ride on mower it would appear option 2 is just wishful thinking. The LiFePO4 cells plus my BMS work out to be initially 30% more expensive than a lead acid battery. The economics strongly favour option 3 if a special BMS is used that compensates for the poor quality of the mower's electrical charging system.
The Special BMS - Requirements
It should be obvious but nothing you can buy off the shelf is really designed to maximise the life of a LiFePO4 battery. All they try to do is prevent catastrophic failure until the warranty period has ended. That sounds cynical but ask the BMS seller if they are confident the LiFePO4 battery will still be good in 30 years. And if you buy a BMS from an online Chinese marketplace then you know your warranty ends as soon as money changes hands.
To get a long life from a LiFePO4 we must avoid:
- charging in freezing temperatures,
- over-charging, and
- discharging below 0% state of charge
Point 1 - charging in freezing temperatures
Point 1 is not applicable in this application since I never go lawn
mowing when it's freezing cold. But it
does mean this might not work in a snowmobile or similar without a
little more thought.
Point 2 - over-charging
This is my main concern given my mower, and probably yours too, has an unregulated charging voltage. In testing I found my mower could readily push the LiFePO4 battery charging voltage over 14.2 volts.
Yes, I read that when the charging voltage reaches 14.6 volts the battery is 100% charged. But in my opinion charging to 100% is an aggressive charging regime that is unwarranted in this application. I'll take the conservative option and try to hold the maximum charging voltage under 14.0 volts. I really want the battery to last and if I get this right I'll be replacing the mower, not replacing the battery, in 20 years or so!
Point 3 - discharging below 0% state of charge
What does it really mean when you read statements like the following? :
"over-discharging can lead to battery damage. If any battery is near empty (below 2.5V), the BMS must be disconnected from the load. Batteries may be slightly damaged below 2.0V, but are usually recoverable. However, a battery driven to a negative voltage will be damaged beyond recovery"
I certainly agree that the 2.5 volt threshold can be a point off failure since I've experienced it first hand. However, in an engine starting application I'm unsure if the individual cell voltage is relevant. Does the 2.5 volt threshold apply under heavy intermittent loads?
I'm going to suggest that this BMS doesn't need to monitor for low cell voltages while cranking. The reason is based on ohms low in a worse case scenario. If I have a cell with 3.2v of terminal voltage and an internal resistance of 5mΩ, then a 150Amp starting current pulse would drag the voltage at the battery voltage down by 0.75V to 2.45V. But the cell has not been discharged. It is just delivering a lot of current. This appears to be just what happens with a lead acid battery.
Hence, in this engine cranking application I will use a buzzer to warn if the battery voltage is below
12.8 volts, or 3.2 volts per cell. I made provision for those who, like me, decide that modifying the mower's wiring is worthwhile to implement a lock-out for the starter solenoid. This prevents the engine cranking when the battery is lower than 12.8 volts.
Interaction of Cell Size and BMS parameters
The smallest cells I tested in my mower were sold as 14Ahr. During testing they performed well but in the final application a slightly larger cell of 30Ahr will be used. It is useful to compare the expected charge/discharge regime in the mower with a sound research paper such as
the one found here.
| |
D. Wong et al. |
14Ahr Cell |
30 Ahr cell |
| Cell
capacity |
2.6
Ahrs |
14.0
Ahrs |
30.0
Ahrs |
| Discharge Current |
40 Amps |
150 Amps ?
|
150 Amps ?
|
| Discharge
Rate |
15C |
11C |
5C |
| Discharge Time |
5* |
3 |
3 |
| Depth
of Discharge |
100% |
0.9% |
0.4% |
| Charge Current |
40 Amps** |
3 Amps |
3 Amps |
| Charge
Rate |
15.38C |
0.21C |
0.10C |
* 5 seconds discharge followed by 1 second recharge. Repeated till 100% discharged
** After fully discharged recharged at this rate
Even with a pessimistic cranking current the larger 30 Amp Hour cells are operating under very different, and I have assumed more favourable, conditions than that tested by D. Wong et al. The tested cells lasted around 47 cycles under the testing regime used by D. Wong et al. Assuming a 1% depth of discharge from cranking that might mean 4700 cranking cycles from the LiFePO4 battery. At two cranking cycles a week that's 45 years of life. I won't be around to verify they last that long but I have a lot of confidence the LiFePO4 battery can outlast the mower.
Proof of Concept Results
No
issues with starting the motor and charging the battery. However, I wasn't
entirely happy with the high voltage (18V) being seen by the hour meter
and fuel cut-off solenoid once the battery was charged and disconnected from the electrical system. A low resistance load resistor solved the
high voltage at the expense of heat.
BMS Prototype 1
My proof of concept design had a lot of smarts but the crud coming from the alternator meant a real belt and braces approach was needed.
I ended up using a BMS with the following characteristics:
- A
number of parallel mosfets which when turned on allow the mower to be
started and the alternator to directly charge the batteries.
- Once
the battery voltage rises above the set point a dump resistor
is switched across the alternator to absorb some of the energy and the mosfets isolate the battery. No further charging takes place.
The only reason for the resistor bank is to keep the fuel solenoid and hour meter from seeing too high a voltage. If you didn't have, or were prepared to risk, an hour meter or fuel cutoff solenoid being damaged then the dump resistor may be unnecessary.
My set point was to stop charging when the battery reached 13.4 Volts. Combined
with a charge balancer it should ensure no damage is ever done by
overcharging the LiFePO4 cells. The state of charge should approach 90%
at most.
BMS Circuit Diagram
I'm reluctant to publish a full circuit diagram because it will be instantly copied by the Chinese and sold with inferior parts which will reflect poorly on me. Contact me if you want to see the finer detail.
Solid state or Relay?
My initial "proof of concept" tests were with a modified Low Voltage Disconnect Switch working in reverse. It utilised 3 90A mosfets and survived a lot of abuse.
When I built the first prototype of this BMS I used 3 x 47 Amp mosfets, of Chinese origin. Magic smoke escaped.
Rebuilt with 3 Chinese x 74 Amp mosfets. Magic smoke escaped.
Rebuilt with Western specified 90 Amp mosfets - success.
I struggle with the idea my mower needs a cranking current over 150 Amps. Replacement starter motors are specified as 0.9kW, or 75 Amps. I will assume 150 Amps until I measure it.
So correctly specified mosfets will do the job. I considered a starting solenoid as a relay, but I'm unsure if the coil can handle being energised for longer than a cranking time period. In any event, a relay or starter solenoid is more expensive so I'll stick with the mosfets for now.
BMS Version 1
Taking everything I learned from the proof of concept and the prototype I produced Version 1 of the BMS. Everything works as intended though using the load resistor to tame the "alternator" is not an elegant solution. But it means no change to the mower wiring is required.
And the resistor gets hot. Initially I had it mounted by standoffs on the plastic case. It melted the plastic and fell off to be left dangling by the wires. It is now mounted behind the seat in the open air.
And here is a picture of Version 1 temporarily installed in the mower with the small LiFePO4 battery used for testing. With the arrival of spring I'll be mowing weekly so this combination will be tested to destruction. I have some more pcb's so I can rebuild it as needed.
After a delay in posting this spring has turned into summer and those baby LiFePO4 cells are still working well. I experimented with the charging cut-off algorithm and the current iteration is a hard cut-off at 13.6V or 3.4V per cell.
BMS Version 2
There are already a couple of points which could be improved upon. Version 2 is taking shape and I'm just waiting to see what might fail in version 1 before I finalise it. I'm not in a great hurry but the goal is to not only replace the mower battery but also the secondary lead acid battery in my 4WD. That battery fails about every 4 years so I have some time up my sleeve to get this right.
