Charging Two Batteries in Parallel

Electrical and electronic topics for small boats
Twofish199
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Joined: Wed Apr 21, 2021 4:02 pm

Charging Two Batteries in Parallel

Postby Twofish199 » Wed Apr 21, 2021 4:20 pm

The engine starting battery on my boat also powers the SONAR, the navigation lamps which have LED lightbulbs, and a small rarely-used LED flood lamp. Two just-addedCannon Mag 10 down riggers are also powered by the engine starting battery

The engine is always running when down riggers are used. The engine speed does not decrease with use of the down riggers.

I want add a second battery.

Is there any harm in connecting a deep cycle battery to the starting battery in parallel?

I'm assuming if the two batteries are connected in parallel they will both be charged by the engine's alternator.

Is this a safe assumption?

The engine is a 2014 Honda 150.

jimh
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Location: Michigan, Lower Peninsula
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Re: Charging Two Batteries in Parallel

Postby jimh » Wed Apr 21, 2021 7:18 pm

Twofish199 wrote:Is there any harm in connecting a deep cycle battery to the starting battery in parallel?

As long as the two batteries are of the same type, for example, both lead-acid batteries, and as long as they are both in good condition, that is, neither has a shorted cell and neither has any internal parasitic discharge, then you can connect two batteries in parallel. When you do this, if the terminal voltage is not exactly the same and the internal resistance is not the same, the batteries won't operate as identical batteries. The battery with the higher terminal voltage and lower internal resistance will tend to discharge current into the battery with the lower terminal resistance and the higher internal resistance.

Twofish199 wrote:I'm assuming if the two batteries are connected in parallel they will both be charged by the engine's alternator. Is this a safe assumption?


When two batteries are wired in parallel and connected to a source of charging current, how the charging current divides between the two batteries is hard to predict. Typically the battery with the lower internal resistance and the lower terminal voltage will absorb more charging current. Since the batteries are in parallel, some of that current could come from the other battery if it is in a higher state of charge and has a higher terminal voltage.

Operating with two batteries always wired in parallel is not a good arrangement. It works, but it is not the best approach.

Charging two batteries by wiring them in parallel is not a good arrangement. It may work, but it is not the best approach.

jimh
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Re: Charging Two Batteries in Parallel

Postby jimh » Thu Apr 22, 2021 10:21 am

On the corollary and unmentioned topic of management of power distribution with two batteries available and for management of the re-charging of the two batteries, the best arrangement will provide for isolation of the two batteries and their loads from each other, and for isolation of the re-charging of the two batteries from each other.

Altering the power distribution wiring so that loads are isolated to one battery or the other is straightforward, and I will not attempt to describe the method here.

Re-charging each of two batteries in isolation is best accomplished if the charging source, typically an outboard engine, can provide two isolated battery charging outputs. Many engines have that feature.

If isolated charging outputs are not available from the outboard engine, the next-best option is to employ an automatically-operated battery combiner switch. These devices are generally sold under the designation of ACR (automatic combiner switch) or VSR (voltage sensitive relay). Such an arrangement typically lets the engine charge its attached cranking battery in isolation until the engine cranking battery terminal voltage rises to a threshold level which is indicative of being at a state of full charge or near full charge. When that voltage threshold is reached, the automatic combiner switch is activated, and the second battery is placed in parallel with the engine cranking battery. This action also places all the loads in parallel, ending the isolation of the loads when the two batteries are not in parallel. The charging current from the engine then will begin to re-charge the second battery while wired in parallel with the first battery.

Of course, such switching could be accomplished without the automated feature by the operator just manually operating a switch that placed the two batteries in parallel at the appropriate moments and separated them again when appropriate. The automated feature is generally based on sensing the voltage of the engine cranking battery.

If the second battery is at a very low state of charge, when it is added in parallel with the primary battery the effect of that combining may cause the terminal voltage of the primary battery to drop below the threshold voltage for combining. When the combing has been ended, the terminal voltage of the primary battery again rises, causing the combing to reoccur. This behavior creates an oscillation in the combined or not-combined state. The better devices use a second threshold voltage to be the decision point for uncombing. In a control circuit this behavior is known as hysteresis, and is an important consideration in creating useful control. In choosing an ACR or VSR, be certain the devices incorporates hysteresis in the combine-threshold and the un-combine threshold voltages.

Some products that provide this function also include more features, such as feature that restores complete isolation of the two batteries and their loads during engine cranking. This avoids creating problems with electronic loads from battery voltage sag during engine cranking.

These products are well-known and are described in detail by their manufacturers, so I will not attempt to describe them further here.

For some prior discussion on this topic, read these earlier articles published some time ago:

DUAL CIRCUIT BATTERY SWITCHES
http://continuouswave.com/forum/viewtopic.php?f=9&t=2627

AUXILIARY BATTERY CHARGING KIT FOR E-TEC
http://continuouswave.com/ubb/Forum6/HTML/002758.html

BLUE SEA SYSTEMS Automatic Combiner Relay
http://continuouswave.com/forum/viewtopic.php?f=9&t=912