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I plan on running a Garmin UHD 73SV fish finder off two 12V 7A lead acid batteries for a kayak. The unit pulls 1.25Ah. So if I run them in parallel I will have 12v 14A with a run time of about 11.2 hours (I will be on the water for 8-9 hours during tournaments). If I run them in series I will have 24V 7A with the same run time. The unit can run off a 12V or 24V system and I will have them wired with 14AWG wire about 4ft. The cables that connect to the unit itself are 16-18AWG and I will solder a connection with the 14Ga wire.

What's the best option and why?

Feel free to correct me if I am wrong, no hard feelings!

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  • \$\begingroup\$ No, the unit pulls 1.25A not 1.25 Ah. Assuming it pulls half that at 24V there is no run time difference between the setups. Both will kill lead acid batteries fast if you are discharging them below 50% capacity so if you need hundreds of cycles rather than dozens, you need bigger batteries - or more tolerant ones like LiFePO4. \$\endgroup\$ – Brian Drummond Mar 26 at 12:53
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Two batteries store the same energy, whether connected series or parallel. If the load can accept either voltage, and deal with it in an energy efficient way using switch mode rather than linear power handling, which it sounds like it can from your assertion that run time is 11.2 hours for either configuration, then there's no difference for run time.

The choice of series or parallel must therefore be made for other considerations.

If the transmission distance was large and wire cross section was an issue, then series will give you lower currents, so lower wiring losses. At your distance with the wires you have available, this is not such a deciding factor. Small vote for series.

Series gives you simpler wiring, your entire circuit can be protected by one fuse and one single pole switch. Parallel needs tee'd connections, ideally each battery is fused separately, and two switch poles to disconnect both batteries. Small vote for series.

Reversion, what happens if one battery dies? As the load can accept both voltages, this can be achieved by connecting just one battery. If the system has to power other loads as well as your flexible input voltage fish-finder, then if they're single voltage capable, it's a strong vote for parallel. If you always have enough batteries to make a full set, then it's no vote.

Cascading failure, can one battery 'take out' the other? That's why we fuse each battery separately with a parallel connection. In a series connection, at end of discharge, the stronger battery could reverse charge the weaker one. Small vote for parallel.

Flexibility, a series connection lets you power either 12 V or 24 V loads. Sounds like a good vote for series. Or does it? Powering a single 12 V load from one of the series batteries unbalances their discharge. For a tiny imbalance it's probably not a problem, but for a significant imbalance, it limits the lifetime you can achieve on the main load, and promotes the reverse charge at end of discharge problem. If you can be disciplined and refrain from using the 12 V centre tap for significant loads, then it's a small vote for series. If you're tempted to use a 12 V light, then it's a bigger vote for parallel to prevent you from unbalancing the batteries.

Different considerations will have different weightings for different people. As you see, there's no strong reason to choose either. If you find one of those reasons suddenly becomes stronger, say you want to power your fish-finder in a remote drone boat, and need the higher voltage to use thinner wires, or you want to run your 12 V TV as well so need parallel, you might as well just flip a coin. You may have to keep flipping the coin until it comes up the way you really want!

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