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I'm considering a DIY project of a jump start kit. I know it's commercially available but I thought it'd be fun to DIY and customize the way I want.

I guess the first order of business is to choose a battery. I have seen some people use lithium batteries and I'm a little skeptical whether they can provide the current as well as the finickiness of dealing with lithium batteries.

A more logical choice is a lead acid battery, which should be easier to deal with. I suppose a 5Ah or 7Ah battery, commonly used in UPS units, should be sufficient for an average sedan or SUV. I'm slightly worried about the battery terminals though. Are they able to handle the kind of current a starter draws? I know it's just a couple of seconds and cheap jump cables only use 14-AWG wires (if not thinner), which can only handle 20 Amps, and they seem to work OK.

Any thoughts and advices? Thanks.

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  • Welcome to Motor Vehicle Maintenance & Repair! When dealing with an automotive battery, it can take hundreds of Amps to provide enough power to start the vehicle. 20A isn't going to do much. You'll end up melting the wiring in your jumper box as the car tries to pull as many Amps as it can get. For instance, the battery in my truck is ~800 cold cranking Amps (CCA). Not that it uses all of that, but that is what is spec'd for it. Your jumper box with 20A wouldn't even come close to that.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Mar 20 at 17:48
  • You have to consider that by the time you've bought an appropriate lead/acid battery & a charger for it, you may as well have got a commercial starter. My jump starter is 1,000A & can start a 3 litre V6 diesel. Li-ion is a good modern structure, but an absolute no-no for DIY. Mine charges over regular USB & can either start the car 20 times or without use last a year between charges. The same manufacturer makes them up to 6,000A, for trucks.
    – Tetsujin
    Mar 22 at 18:00

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5-7 Ah is not adequate. I have a no-frills jump starter about 20 years old with an 18 Ah 12V sealed lead-acid battery that works well. This size battery is widely used in larger UPSs. I replace the battery about every 5 years. I charge it every 6 months unless I use it to jump-start someone; then I recharge it immediately.

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The cables are AWG #4 stranded copper. Suggest you use that size or heavier. Thin 14 gauge wire can deliver current to charge a depleted battery if you leave it connected long enough, but won't start the engine. AWG 4 or heavier can actually start the car immediately with no waiting.

The battery will have no problem delivering whatever amps it's rated for through its terminals, provided you use a nut and bolt with washers that adequately clamp the ring connector that you install on your cables.

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  • 4AWG minimum with pure copper stranded and NOT copper covered aluminum (CCA).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Mar 20 at 20:40
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Are you aware of jump starting using both main battery and boost battery for starting? As long as the main battery has any charge left in it, it will add to the boost battery output amperage and may fool many into thinking a jump battery is doing all the work. No one has tried starting with main battery disconnected and using only the jump battery for starting. Guess why? Jump batteries changed from heavy lead acid, sealed lead acid to very compact lithium rechargeables. Do you know what your starter amperage draws? In freezing temperatures where more current is needed whereas warm summer temps draw less current? The 12v 7AH battery, according to one manufacturer has a maximum output of 108 amps for 5 seconds. The 12v 18AH battery maximum output is 255 amps for 5 seconds. Lithium jump packs are more powerful with max output listed in small packages.

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