Connecting fully charged lead acid to dead lead acid. - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: April 26, 2020, 11:22:26 am
content: So obviously people do this every day when jump starting one car with a dead battery from another with a running engine.My basic understanding of lead acids is that they will theoretically pull huge current when not close to fully charged. Thus their charging profile is constant current, then constant voltage to "top off".This would suggest that when you connect a battery at 14.40V to a battery at 9V huge current would flow. But how much current?I expect a lot of things will come into play as the current rises the voltage drop over the jump leads will eventually limit it there. That could be 100s of Amps though.Pulling the voltage of the donor car down as the current rises sounds plausible, but you maybe have a 40Amp alternator and a lead acid capable of supplying 400-500 Amps and if it's fresh it will still register over 10 volts doing so.The receiving battery will no doubt heat up, it's electrolyte boil and bubble and it's internal resistance rise. Surely that going to harm it though.So my questions are...1. Just how many amps will flow with two lead acids in parallel if one is "flat" and the other "on full charger and actively charged".2. How much damage will this do to the dead battery? Flattening a lead acid is damaging enough, but car batteries tend to go from bad to worse rapidly. I wonder how much of this comes down to the damage done during jump starting.3. Would it better to limit the amount of time they are connected, as in, connect, start, disconnect rapidly... or leave it for the two batteries to stabilise and the current to drop off before cranking?

author: StuartA
author_rank: Regular Contributor
timestamp: April 26, 2020, 12:45:37 pm
content: The wonderful thing about car batteries is that you ask 10 people for their insight - and get 12 different replies.The thing to remember here is that batteries are not like capacitors, where you get near-instantaneous balancing of voltage. The battery has to do chemical reactions to receive or send charge and that takes some time, and also, you get this complex boundary layer of electrolyte against the plates where reaction products are coming and going, and all this limits the rate of flow of current. You get this "over-voltage" effect in a battery being charged where the boundary layer effect makes even a deeply discharged batteryshowa much higher voltage than is really "true". Disconnect it for a couple of minutes and the voltage will very quickly fall as the boundary layer disperses.Don't you have a charger (most bench psu's will do)- takes longer , but the battery will thank you for using it?

author: Psi
author_rank: Super Contributor
timestamp: April 26, 2020, 12:59:16 pm
content: The source battery voltage sag from the huge current draw, plus the voltage loss in the jumper cables resistance, means that the current is somewhat limited to sane levels.It's still not all that good for the battery, especially the flat one.but you're not jump starting for very long, and lead acid car batteries are designed to handle huge bursts of current.Trying the same thing with super capacitors is another story.

author: StuartA
author_rank: Regular Contributor
timestamp: April 26, 2020, 01:54:45 pm
content: Quote from: Psi on April 26, 2020, 12:59:16 pmThe way round that problem is to have your jumper cables "up-graded" by your buddy in the workshop of one of the worlds biggest defence companies

author: tpowell1830
author_rank: Frequent Contributor
timestamp: April 26, 2020, 02:02:46 pm
content: I agree with Stuarta's explanation.So, with jumper cables you have a large gage wire, but typically they are 8 AWG, which is safely capable of continuous current of about 50-60 amps. With this current, the cable will get very hot. Now, look at the typical method of connecting the jumpers to the battery: a spring loaded clamp made of stamped metal. This only contacts a very small area when connected. Overall, the current is limited with jumper cables.Next, let's take a scenario where the dead battery is at 9 volts. Connecting a set of jumper cables as described above will only allow, through physics, 40-60 amps (depending on connection points). This means that, unless your starter motor will spin at 50-60 amps, you will not be able to just connect and start the engine with dead battery. So, what I do is I leave the donor vehicle running, which allows the alternator to continuosly charge BOTH batteries for, oh let's say five minutes before trying. This will allow the dead battery to be charged and help the current output to the starter. That is my standard method for jump starting.However, if you are connecting a dead battery to a fully charged battery with a large conductor using standard battery clamps, your current between the 2 batteries will be very large, whereas with a jumper cable, there are limiting factors, i.e. size of wire, connection point area.In response to your questions:1. It depends on the method in which you connect the 2 batteries.2. In regards to jump starting, I would say the damage is insignificant if the dead battery is in good condition (the lead plates are not deteriorated)3. You will almost be required to allow the dead battery a short time to charge at 35 amps so that it can help more during starting. Typically, starters will pull from 90 to 200 amps, depending on the size of motor and the physical condition of bearings, etc.This is just a generalization, your results may vary.Hope this helps...

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2020, 03:17:53 pm
content: The relatively high resistance of the jumper cables and the higher than normal internal resistance of a discharged battery limit the charging current to safe levels.

author: StuartA
author_rank: Regular Contributor
timestamp: April 26, 2020, 04:24:35 pm
content: Quote from: David Hess on April 26, 2020, 03:17:53 pmDoes that mean that jumper cables come under the Goldilocks condition then? Too little resistance and they would be unsafe, too much resistance and they would be ineffective? To be good, they need to have just the right resistance?. I wonder if there is a MIL-spec for jumper cables and does it specify the acceptable resistance range?I have it on good authority that the Apollo 11 lunar module carried some jumper cables. If the ascent engine failed to operate, there were various fallback options to get it to work (very likely, a solenoid valve issue) including patching power from a battery not normally connected to the propulsion system.Of course, I'm not saying these were anything like auto-jumpers, but jumpers nonetheless.

author: paulca
author_rank: Super Contributor
timestamp: April 26, 2020, 05:03:51 pm
content: I literally last week just started my car with a dead battery using nothing but a fully charged marine battery on the end of jumpers. It was a little sluggish, but it started. Car was stone cold too.Though my jumpers are fairly high quality ones, wires are nearly 10mm thick, including insulation.

author: StuartA
author_rank: Regular Contributor
timestamp: April 26, 2020, 09:18:31 pm
content: Back in the days when you could leave your headlights on, and then open the door and get out of the car all without any alarm going off, most people experienced a bad day when, on finishing work, they returned to a dead flat battery. Seeing people with jump leads in the car park was common.