Can a Car Battery Charge Another Battery? [Here’s How!]

Gaston Planté was a French physicist born in France on 22 April 1834 and was responsible for developing the first viable lead-acid battery power system capable of being recharged. Since then, more than 94 battery technologies have been invented, 62 of which are rechargeable.

You can charge a donor battery from a car battery; however, if you intend to use the car’s electrical system to keep the donor unit charged, it will mean running your engine for many hours. Removing the battery from the car and connecting them will charge the recipient battery.

If you are stuck and have no way of charging a dead battery using a recharger, it is possible to use the car battery to charge the dead unit. You will need to check out several factors before carrying out this procedure.

How to Use a Car Battery to Charge Another Battery?

There are two possible scenarios when charging a recipient battery from a car battery,

The car’s battery can be connected to the recipient battery while in the car, and the engine is running, enabling the alternator to maintain a charge. The problem is that it could take a long time to fully charge the recipient battery, which will be unpleasant and very expensive with current gas prices.

Because of this, the discussion in this article assumes the donor battery will be outside of the car.

How Do You Charge Another Battery With The Car Battery?

The most viable method to recharge a battery is if you have a battery charger.

If the circumstances are urgent and there is no alternative, you can charge another battery with the one in the car as follows.

Step One – Disconnect The Cars Battery

Disconnect the car’s ground battery cable, then its live battery cable, and move these out of the way.

If you have a voltmeter, check the state of charge of the car’s battery (when it is disconnected), and if it shows a healthy charge, you can proceed.

Step Two – Connect The Two Batteries

If the battery chambers have removable lids, take them off and store them in a safe place (not on the car’s paintwork).

Attach a cable from the negative (-) terminal (red) on the car’s battery to the negative (red) terminal on the recipient battery.

Attach a cable from the positive (+) terminal (black) on the car’s battery to the positive terminal (black) on the recipient battery.

Step Three – Start Charging

The battery will start charging, and if you want the maximum possible charge, it could take between twelve and twenty-four hours.

Use a voltmeter periodically to check both batteries’ state of charge and refer to the table below to match the voltage against the estimated charge.

When the voltmeter reading is equal on both batteries, the maximum charge percentage has been reached.

Limitations to Keep in Mind

A battery can only use its available charge to recharge another battery.

An analogy is to connect a pipe to join the bottom of two equally sized water jugs. Let’s assume that one of the jugs is full and the other empty and that the connection between them is watertight.

The water in the full jug will fill the empty jug to the point where both have equal water levels. The (previously full jug) will now be half full, and the (previously empty jug) will have filled up to half full.

Assuming the “full” battery has a 100% state of charge and the “empty” battery has a 0% charge, the final charge in both batteries will be 50%.

While this may sound like a workable solution, different battery types can only be discharged to a certain level, after which their life expectancy is compromised.

How Does Condition of Donor Battery Affect Charging Another Battery?

Two terms are related to the battery’s charge state and life expectancy.

  1. State of charge (how close to fully charged the battery is).
  2. Depth of discharge (how close to fully discharged the battery is).

An inverse and proportional relationship exist between the discharge depth and the charge state. Additionally, as the state of charge decreases, the voltage produced by the battery reduces.

Knowing this helps determine what percentage charge the battery holds.

A reduction in voltage of a typical twelve-volt battery is listed in the table below.

State of ChargeDepth of dischargeLead Acid Battery Voltage producedLithium-Ion Battery Voltage produced
100%Fully Charged12.74 volts13.60 volts
90%10%12.62 volts13.32 volts
80%20%12.50 volts13.28 volts
70%30%12.38 volts13.20 volts
60%40%12.24 volts13.16 volts
50%50%12.10 volts13.13 volts
40%60%11.96 volts13.10 volts
30%70%11.82 volts13.00 volts
20%80%11.66 volts12.90 volts
10%90%11.50 volts12.00 volts
Discharged100%0.00 volts10.00 volts

Increased Depth Of Discharge Reduces The Life Expectancy

If the donor battery has less than a 100% state of charge, the higher the chance that the life expectancy will be compromised.

The maximum depth of discharge each battery type can tolerate is as follows.

Battery TypeDepth Of Discharge  ToleratedLowest State Of Charge Tolerated
Flooded Lead Acid batteries50%50%
AGM batteries70%30%
Gel Cell Batteries25%25%
Lithium-Ion Batteries85%15%

When using one battery to charge another, the final charge between the two batteries is the average charge between the two.

It could result in the following scenarios.

Scenario One

  1. 1.     The donor battery has a 100% state of charge.

Scenario Two

  1. has a 50% state of charge.
  2. The recipient battery has a 0% state of charge.
  3. It will result in both batteries ending up with a 25% charge.

Scenario Three

  1. The donor battery has a 60% state of charge.
  2. The recipient battery has a 0% state of charge.
  3. It will result in both batteries ending up with a 30% charge.
  • A Lead Acid battery will be compromised in all three scenarios because its final charge will be 50% or less.
  • An AGM battery will be compromised by scenarios two and three because its final charge will be 30% or less.
  • A Gel Cell battery will be compromised by scenario two because its final charge will be 25% or less.
  • The only battery which can be safely used in all three scenarios is a Lithium-Ion Battery.

Can You Charge Multiple Car Batteries At Once

In theory, it is possible to charge multiple batteries simultaneously; however, two factors must be considered.

Charge Must be Balanced

The battery connections must be correctly made to achieve a balanced charge.

If you do not connect the batteries correctly, one donor recipient battery will be favored over the other.

To correctly connect the batteries.

  1. Attach a cable from the negative (-) terminal (red) on the donor battery to the negative (red) terminal on the first recipient battery.
  2. Attach a cable from the negative (-) terminal (red) on the donor battery to the negative (red) terminal on the second recipient battery.
  3. Attach a cable from the positive (+) terminal (black) on the donor battery to the positive (+) terminal (black) on the first recipient battery.
  4. Attach a cable from the positive (+) terminal (black) on the donor battery to the positive (+) terminal (black)  on the second recipient battery.

The Charge Will be Distributed Amongst Each Individual Batteries

The available charge in the donor battery will be distributed amongst more than one battery.

As explained in the section above, which discussed how the condition of the donor battery is affected, if more than one battery is attached, the consequences are more serious.

If the donor battery has 100% charge and is connected to two dead batteries with zero charges, all three batteries will end up with a 1/3rd charge.

How Long Does it Take to Charge a Car Battery from Another Battery?

The length of time the donor battery will take to charge the recipient is determined by the amp hour rating of the battery.

The higher the amp hours of the donor battery, the faster it will charge, which is determined by the following formula.

(Recipient Battery Capacity (AH) *% Depth Of Discharge )/ (Donor Battery Capacity (AH) * Depth Of Discharge) = hours

Assumptions

Recipient battery Capacity – 60 amp hours

Depth Of Discharge  % – 10%

Donor battery Capacity – 60 amp hours

Depth Of Discharge  % – 100%

Calculate Time Taken

60 Ah * 90% = 54 AH

54 Ah / 60 AH = 0.9 hours (54 minutes)

How Do You Charge A Battery Outside The Car?

If the circumstances are urgent and there is no alternative, you can charge another battery with the one in the car as follows.

Step One – Disconnect The Cars Battery

Disconnect the car’s ground battery cable, then its live battery cable, and move these out of the way.

If you have a voltmeter, check the state of charge of the car’s battery (when it is disconnected), and if it shows a healthy charge, you can proceed.

Step Two – Connect The Two Batteries

If the battery chambers have removable lids, take them off and store them in a safe place (not on the car’s paintwork).

Attach a cable from the negative (-) terminal (red) on the car’s battery to the negative (red) terminal on the recipient battery.

Attach a cable from the positive (+) terminal (black) on the car’s battery to the positive terminal (black) on the recipient battery.

Step Three – Start Charging

Use a voltmeter periodically to check both batteries’ state of charge and refer to the table below to match the voltage against the estimated charge.

When the voltmeter reading is equal on both batteries, the maximum charge percentage has been reached.

Can You Charge A Car Battery Without Disconnecting It?

It is possible to charge a battery without disconnecting it.

You must always connect the ground wire (Black (+)) before connecting the live wire (Red (-)).

How Do The Different Car Battery Technologies Work?

The  most commonly used batteries in modern cars are

  1. batteries
  • Lithium-Ion Batteries

Flooded Lead Acid Batteries

Flooded Lead Acid Batteries consist of tightly packed alternating sheets of

  1. Spongy or porous lead (anodes) 
  2. lead oxide (cathodes).

The plates are placed in alternating rows in the battery casing. The battery casing is filled with an electrolyte (made up of water and sulfuric acid solution).

The lead gives up electrons (electricity) which the lead oxide accepts and turns the plates into solid lead sulfate.

As the layer of lead sulfate increases, the voltage decreases, and the battery discharges, if it is not quickly recharged, the lead sulfate crystallizes, and the battery will be compromised.

AGM Batteries

AGM batteries use the same technology as Lead Acid Batteries with two specific differences.

A Fiberglass Mat Is Positioned Between The Sheets

The mat absorbs electrolyte (made up of water and sulfuric acid solution) and prevents it from flowing freely inside the battery.

The Sheets Are Compressed Into A Smaller Space

The sheets are compressed, which reduces the shedding of solid lead sulfate caused by the discharge and recharging process. It results in a significantly longer life.

Gel Cell Batteries

Gel Cell batteries replace the liquid inside the battery cells with a special silica gel that holds the electrolytes together.

The material allows the electrons to flow freely from the plates.

Lithium-Ion Batteries

A Lithium-Ion Battery consists of the following components.

  1. An anode
  2. A cathode
  3. A separator
  4. Electrolyte
  5. Positive and negative collectors

Lithium-Ion Batteries create electricity by the physical movement of electrons within the battery.

The cathode and anode are used as storage devices for the electrical current. An electrolyte carries the positively charged lithium ions from the anode (negative electrode) through the partition and across the positive collector to the cathode (positive electrode).

Lithium is deposited when the ions and electrons combine at the positive electrode.

When all the ions have moved from the anode to the cathode, the battery is fully discharged and needs to be charged again.

When a Lithium-ion battery is recharged, the whole process is reversed.

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