On a recent trip to the local supermarket I found a Lawnmower on special offer, 50% off. In the market for a new lawnmower a reduction from 430€ to 215€ was too good an offer to refuse.
However, the reason it was on offer was that it was a Briggs & Stratton electric start model, and they had lost the battery and charger! I purchased it on the spur of the moment, thinking that I would buy a battery and charger and still make a huge saving.
I hadn’t banked on the cost of a replacement battery and charger costing in the region of 140€!
I decided to try to adapt one of my existing power tool batteries to do the job. On investigation I found that a mini hedge cutter from Lidl had a rechargeable battery pack that was very similar in voltage and current as the Briggs & Stratton model.
When testing the Lidl battery I discovered another problem, the Briggs & Stratton InStart battery has 3 terminals. Positive, negative and a third terminal which serves as a switch for the start button (or key). When the positive and negative of the replacement battery are connected without using the third terminal the starter motor turns continuously.
I was going to need to find a way around this issue
I decided to model the battery adaptor in two parts, one part would slot into the lawnmower battery holder and the second would hold the Lidl battery.
The main reason for producing in two parts is to facilitate the printing process. As one unit, there would need to be a significantly larger amount of print support needed.
The first step was to model a replica of the InStart battery pack. Taking measurements from the lawnmower I modelled a block which gave access to the positive and negative pins. Also incorporating a large hole to accept the adaptor for the Lidl battery
The second part of the design was a holder for the Lidl battery which would slot into the first model as shown below. This model has a central cylinder for the spring and holes for the electrical connections.
The Main Block
Printed in PLA upside down with a 0.2mm resolution and 20% infill. Adding support for the two side slots.
Again printed in PLA with a 0.2mm resolution and 20% infill. Support added for the height location ring.
The design and print files can be downloaded on Thingiverse.
To make the connections between the Lidl battery and the Briggs and Stratton terminals I used some copper strips salvaged from an old light fitting adaptor.
As well as the inserting the copper terminals, I added a 30mm long x 6mm diameter compression spring into the center hole.
The copper terminals are then soldered to finish the unit.
The YouTube clip below shows how it works. The battery tends to bounce around a bit if left in the adaptor, so I take it out while using the mower.
Other than that, it works really well.