Tuesday, February 1, 2022

Replacing lead acid battery with lithium battery


This weekend I changed the battery on my lawn mower to a lithium battery. Not counting the research and preparation, it was about 2 hours of work. The new battery is 130lbs compared to 260 lbs of the old battery. I expect the usable range of the mower to triple with this swap.

When I bought this mower, I knew I would have to change it at some point. Lead acid batteries are really good at short term surges, which is why we still use them in our cars for starter motors. But they are terrible at long term use. 

Ryobi did release a lithium battery mower an year after I bought my mower, but when I looked at it, it's an 80 volts system and very complicated with multiple battery stacks and would be a nightmare for balancing, troubleshooting, or replacing batteries. Hence I decided to just upgrade my 48V system.


Theory and removal of connections

There are three critical parameters that need to match for a good battery replacement.

1. Voltage- this is simple. A 48 volt mower can only take a 48 volt battery.

2. Current- current is how much power a machine draws while operating. Lead batteries are ridiculously good at current surges. I didn't even try to find out the current rating for my old batteries. All I needed to know was how much demand does the mower place on this battery. Hence the calculations below.

Since my 115AH system ran for about an hour before dropping to 50%, my rough estimate is it draws less than 60amps at steady state. 

115AH * 0.5 (50%spent) = x amps* 1hr

=> x = 115 AH*0.5/1 H = 57.5 amp

However, initial surge of current can be multifold when a machine starts from standstill. Therefore I chose a battery with a higher rating that can provide 150amp in steady state, 300amps for 10 seconds(Short term) and a whopping 1000Amps for surges.

3. Total capacity- My old battery was 115AH. I decided to go with 150AH. With larger operating range and higher capacity this should triple the total capacity.

Amp-hr basically tells you how long the battery will last at a certain current draw. A small motor that only draws 1 amp will last 150hrs. A large motor that draws 75 amps will drain this battery in 2 hrs. And so on.

Multiplied by voltage this gives you total energy available. A 150AH at 48volts a total of 48*150 = 7200 watt-hr. 

Or 7.2 kwh. 

Another battery at 96 volts and 75 AH would also have the same amount of energy available, but delivered at double the voltage.

Removing the old battery

Putting new battery in

Repurposing old charger port for new Li-ion battery charger

The power surfing project

  01-Apr-2023: Sometime ago I had posted a video of a 400w trolling motor on my paddle board. It's a good option for a leisurely stroll ...