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For various reasons, I keep several very large four cell lithium-polymer batteries in my vehicle. Each of these batteries is rated for over 100 amps of constant current, with 10 second burst ratings of twice that. Wired in parallel, they could easily provide enough power to start my vehicle.

However, a fully-charged 4s LiPo is normally 16.8 Volts and the electrical system on almost all of my vehicles is 12 Volts.

All of my life, I've been told that providing more than 12 volts to any modern car will damage the sensitive electronic components. Obviously, this can't be entirely correct, as a typical alternator can provide up to 15 Volts. I've also seen videos of people using 4s LiPos much smaller than mine to start their cars. I even know one shop mechanic who claims he used to jump 12 Volt vehicles with 24 Volt systems.

So, essentially, I'm wondering what the maximum safe voltage is for jump starting. Would 16.8 Volts be safe? What about a "storage charged" 15.28 Volt battery?

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I'd say that 16.8 VDC is a bit too high to call "safe."

Now to answer the primary question, obviously it depends on the exact vehicle. Different computers and different wiring designs will result in different characteristics. As an example, if your battery is very dead, then putting a 15 VDC trickle on it will read something like 8-10 VDC. You know it's outputting 15, but the battery is soaking up a good bit.

There are many ways that different components and designs can create a buffer effect, but it's not really safe to plan with that.

That said, you can judged what is safe by taking a read off you battery after a 30 minute drive (assuming your battery is healthy and recharge system is working properly). This will be the reading of what you alternator is pushing. For my motorcycle for example, it will be ~14.8 VDC immediately after a ride. This will settle to somewhere around 14.5 I think. So I'd say, in most cases (but not all), that 15 VDC would probably be the max that I'd go.


As a separate note, this mechanic that used 24 VDC was most likely having a lot of juice sucked off by the bad battery. Even in non-digital cars, various relays and fuses are quite sensitive to voltages - not just light bulbs as Trond Hansen suggested.

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i think the absolute max voltage that is farly safe is 14,4 volt,whith todays modern cars and all the computers and sensitiv elecrtonic always use jumper cables that have inbuilt protection.

the reason your old mechanic was able to use 24volt to start a 12 volt car was in the past cars did not have sensitive electronic,the worst ting that happened was some blown lightbulbs.

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While I have lots of opinion, I know of no standard with respect to acceptable voltages to components such as ECUs. Each manufacturer is different. Some ECUs are designed to operate on "28VDC" obviously for markets other than US cars.

In general an alternator will provide up to 14.8VDC when charging a lead acid battery. The voltage is slightly different for AGM. That said, the normal charge voltages are more like 13.8VDC.

Is 16.8VDC too high? It varies, and it is dependent upon your vehicle, it's appliances (GPS, radio, lighting systems, HID/LED system and of course ECU), as well as the vehicle you might connect to as a good Samaritan. Keep in mind that replacement parts may not have the same Vmax rating as the original equipment.

The old days of hooking up a 24VDC jump start system are over, unless one likes replacing an ECU or two every now and then.

So to address your question, you would have to obtain a Vmax spec for each of the components on your vehicle to be certain. Good luck with that research job.

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