# DC Voltage Step-down at High Current

I've asked this question on Electrical Engineering SE but no useful answers there - thought I would try it here instead and see if anyone had a creative solution.

I have a petrol mower with an electric-start engine. The electric start is powered by a small battery of similar size and design as you might find on an electric drill or other power tool, charged by a mains-powered charger, also very similar to a drill battery charger:

After a few years of faithful service, the battery has done what I always knew was going to happen and will no longer hold a charge. Unfortunately, it is no longer possible to buy a replacement battery.

It occurs to me that a might be able to jerry-rig an arrangement that allows it to start by using one of the many drill batteries that I already own.

The problem is that the drill batteries are 18 V - and the mower only needs 10.8 V. I have successfully “jump started” it with a 12 V car jump-starter, so 12 V is a reasonable target to aim for I think.

There is a Youtube Video of somebody successfully modifying his mower for an 18 V battery, by using a cheap buck convertor. He seemed to get away with it, in spite of not really considering the very high current flow that is needed for this application.

My thinking is that the 2 Ah battery will turn the engine for about 1 minute with a full charge, maybe 2 minutes at most. So I believe the current draw is between 36 and 72 A. I haven’t seen any buck convertor that will handle this kind of current.

Is there a cheap/easy way for me to step down 18 V to 12 V at this kind of current draw?

I know I can buy a holder for the battery, so ideally I would like to include all the components on the mower with the holder, so it can be used exactly as the original battery was. I.e. the components should also be able to withstand the harsh vibration etc. of being hot-glued/epoxied to a petrol mower.

The solution doesn't have to be very efficient - if it wastes 50% of the energy in doing the conversion, this is not really an issue.

Notice that the battery pack will have four or five electrical contacts.

Put a volt-meter across various pairs and you'll find that there are different voltages available.

If you are lucky, one of the combinations will be close to the 10.8V target, and you can use that to power your starter motor.

Whether continual use of only one of the sections will have a long-term effect on the lifespan of the battery, I don't know.

Update:
I just checked my DeWalt "20V MAX" battery, and it provided 4.04, 8.08, 12.0, 16.12, and 20.0V.

• I was really excited about this answer, and I think it may well be a great answer depending upon what battery packs other people have. In my case, the batteries have FIVE connections, but, unfortunately, they don't seem to pick-off any subset of the cells at all - measuring every permutation gave me either 20+v or 0v. It would have been so good if this had worked. I suspect you are right that it might cause damage to the battery over the long term - but I have 3 of them and they are easy and cheap to replace. Commented May 5 at 12:52

Another answer, a solution that might actually work.

If the mower start needs 72 A for 3 seconds at 12 volts, it's 216 coulombs. A capacitor of 20 farads ought to be enough, since it stores 240 coulombs at 12 volts.

Buy five 100-farad 2.7-volt supercapacitors and put them into series. This creates a 20-farad 13.5-volt supercapacitor. Then buy a cheap DC-DC buck converter that charges them with 10 amperes.

Such cheap DC-DC buck converters are available from AliExpress from China at very cheap prices. A 24-volt-to-12-volt buck converter in an aluminum heatsink housing works fine, since it can operate from 18 volts as well (unless it has low voltage detection with voltage limits set for a 24-volt lead-acid battery system).

The supercapacitors should be available from AliExpress from China as well.

The only trouble in this case is that you need some soldering skills to make the supercapacitor bank (5 supercaps in series). But this supercapacitor bank should give enough power to start the engine, without needing a 70+ ampere DC-DC buck converter.

The only problem in this case is that you need to wait up to half a minute for the supercapacitors to charge before you can start the engine. If this is a problem, you can of course buy a bigger 24-V-to-12-V DC-DC buck converter capable of outputting 20 amperes.

• This is actually genius - I think it might be the solution that I try putting into practice. It has prompted me to do a bit of Youtube research into capacitors - and my newbie question: Would it be possible to cut-out the buck convertor completely, have, say, TEN caps in series, then "tap-off" 5 of them to give my required voltage and instant recharge? I know the 10 caps will have a capacity lower than your original 5, but is the principle sound? What size caps would I need to use in this scenario? Commented May 10 at 13:40
• Ten caps in series and putting voltage from five of them halves the voltage, but has the problem that the capacitor charges become nonequal. This means that if you repeat the exercise of starting the engine, some of the capacitors increase above their rated voltage and blow up. If you add small resistors to slowly leak the charge, maybe you could make it work if you don't try starting engine twice from the same supercap bank in a very short time. However, I would just buy the buck converter, 10A or even 20A ones aren't expensive. Commented May 11 at 6:54
• Thank you. I am definitely going to explore this. I can buy a supercapacitor "bank" from Amazon where they are all soldered to a nice board for me: 20F/16V. I will get one of these and try charging it with a 12v mains P/S first, just to prove the concept, then work out how to wire a switch that can take this kind of current etc. Commented May 12 at 9:23
• If it doesn't work, most likely the capacitance is too low or the current-handling capacity of the soldered bank isn't good enough. Both can be solved at the same time by putting a second 20F/16V capacitor bank in parallel and soldering them together, and using thick wiring for all the common wires where current from both capacitor bank is flowing. Commented May 12 at 17:35
• Given that the guy in the video manages to start his mower using just a fairly low power buck convertor with NO extra assistance, I'm willing to bet that one of these will be more than enough. Commented May 13 at 7:23

Lithium-based batteries can be a nominal 3.7v (LiFePO4) to about 4.1v (LiPo). Your reference to 10.8v is close enough to 10.1v to consider that your pack is going to be 3 cells of the non-LiFePO4 type. If it is no longer functional, you can crack the housing open and check the chemistry and cell type.

18650 cells are pretty common in the 2.0 to 2.5 Ah range and can be purchased from a multitude of online sources. Batteries that are expected to have high current draws (as in a starter) will have welded interconnects, rather than spring loaded or simple pressure contacts.

I have an equally obsolete battery pack and was able to find a custom builder in Tallahassee, FL, who received my dead pack and returned it with the BMS and new cells for a darn good price, perhaps better than the cells alone. I found the resource on Endless Sphere forums message number seven. After initial contact, he provided a different email address and the project was completed in short order. I was hesitant to send oodles of bucks by Zelle, but he's shown himself to be trustworthy.

If you crack open the housing and find that you can separate components down to the cell level, it will simplify things for the builder.

• Thank you for this. As it happens I have opened it up and the cells don't seem to be the problem. They are indeed 3 x 3.7v, but not 18650, something like 21265..? Anyway, there are a few companies that seem to be in this business in the UK, they don't reply to my emails when I ask if they can help - so this seems to be a (pardon the pun) non-starter. Having this specialist battery pack has always been a problem anyway. I really want to try to get to a point where I can use my other battery packs for this mower, 1) they are just better, 2) I have 3 of them 3) they are always charged Commented May 4 at 17:55

No way is a buck converter an ideal solution. Cranking an engine needs lots of amperes. The problem in this case is that the buck converter would need to withstand ideally at least 30 amperes if not more. While you can buy such buck converters, they are expensive. It's a waste to have 30 amperes worth of power electronics that are being used for only a second at a time.

The best way forward is opening your existing battery and seeing if you can re-cell it.

Another option would be to slightly increase the voltage and use a 12 volt AGM battery. I don't believe that 12 volts will harm your mower, since a 12 volt AGM at full charge is 13 volts. Your "10.8 V nominal" battery probably can hold 13 volts if fully charged, and the starting motor has to withstand that so it certainly can withstand a 12 volt AGM.

The problem with an AGM is that they slowly leak charge and don't like being stored except at nearly full charge, so you need to charge it often. As an example, Fulbat 12V 7.2Ah battery costs around ~20 EUR/USD/whatever and has 105 A max discharge current for 5 seconds, but at that current the voltage sags so much it may not start your engine if it requires 105 A (but it probably doesn't since the drill-style battery is very small and probably can't provide 105 A).

Also a 7.2 Ah AGM weighs 2 kilograms! So that's why you can understand they used li-ion and not lead acid.

• Thank you. As I mention in the question. I am able to start it with a car jump-starter pack that outputs 12v - but I don't want to be stuck with this solution for the life of the mower. I want to be able to use my drill batteries which are always charged and ready to go. To some extent, the cost of the components is less of an issue than the convenience of the outcome. I was prepared to spend £70 on a replacement battery, so if I can use my other batteries and £70-worth of components, I have a better solution for the same money. Commented May 5 at 13:00