Marine battery recommendation?

One of our annual events is the local 4th of July parade, and it’s a long enough route that we end up needing to do a couple battery changes (which can be annoying while on the move, especially if it happens to be needed in the two spots with announcers and larger crowds). I’ve heard of folks using marine batteries for different purposes (combined with an inverter: aux pit power, battery cart power to keep drive station charged, etc.), and it seems like a no-battery-change parade might be another good use.

I can certainly search & find marine batteries for sale, but I’m wondering if there are folks knowledgeable about such things who might recommend a good model or at least a brand. Thanks.

Most common in marine use are flooded (i.e., spillable – not recommended) lead acid batteries, which contain tanks of sulfuric acid and lead plates, or bigger AGM (i.e., nonspillable) lead-acid batteries, which are similar to what the robots already use. You can buy ones that are effectively just more-capacious versions of the competition-legal batteries (e.g., 26Ah, 35Ah). The challenge, regardless, is fitting one in a robot that was designed for an 18Ah battery, and contending with its weight.

You can also get lithium (LiFePO4 or li-ion) marine batteries, but those are heinously expensive, and I would not recommend it.


Thanks for the response. How about sealed gelled electrolyte? Specifically looking at this one by MK:

8G24_Spec_Sheet.pdf (

I like the look of this one because…

  • it’s about 4x an FRC battery’s capacity, but it’s not HUGE like some of the marine batteries.
  • its depth dimension is roughly the same as (slightly smaller than) an FRC battery’s height dimension. This year we laid our battery down on its side, and I think this battery could sit upright in the same spot. We would just have to adjust our arm’s home position to be somewhat higher.
  • it seems like a good size/capacity to put on a robot cart or use in the pit in the future

What about wiring multiple batteries in parallel.

edit: this is probably not safe and should not be done.

edit 2: Research and do this properly, and it should be safe. Other people have detailed some precautions that you should follow.


Gel-cell batteries aren’t very common nowadays; they’ve mostly been replaced by AGM. My guess is that this would work just fine on the discharge side, but I know that you’ll need to be careful when recharging it – you’ll need a special gel-cell charger or a charger with a gel-cell mode.

You may want to install some MRBFs between the battery terminals and the Anderson pigtail, just in case something…happens.

Also, for more life out of the battery during the parade, you might consider limiting your motor currents to a lower level than you would during competition.


It looks like they make virtually the same model as an AGM battery rather than gel, which seems like a wiser choice based on your earlier comment. I assume we might be able to use the same charger. Thanks again!

8A24_spec.pdf (

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Purely out of curiosity as someone just now learning about batteries for hobbyist/gradschool robotics, why wouldn’t this be safe?

(Assuming you took normal precautions to secure each battery properly and such)

You’ll still want a higher-current charger than the 6A ones used for competition batteries – check datasheets or contact MK to be sure, but it’s probably about 0.2C (where “C” is capacity in Ah e.g., 0.2(79Ah) = 15.8A).

It is quite common in marine applications to have 2+ batteries wired in parallel, to leave them paralleled while unattended, or to sleep right next to these paralleled batteries without being concerned. Lead-acid is remarkably forgiving – there’s a reason why we were able to develop it (ease-of-charging) and why it’s still being used (robustness). That said:

  • the batteries are rarely subjected to mechanical shocks comparable to that which an FRC robot sees
  • batteries in a parallel bank are usually left attached to each other for their entire lifespans (or split out, used independently, and never reconnected), and see similar charge and discharge currents – they should have similar capacities and state-of-charge to each other as a result (not guaranteed with used robot batteries)
  • state-of-charge is usually monitored with marine batteries (e.g., coulomb counting) to reduce the chances of over-discharging, and the loads on a boat’s house battery aren’t usually as extreme as FRC batteries (which are more similar to, like, cranking your engine every few seconds)
  • it’s important to wire the batteries such that each one sees a similar total wiring resistance, or the batteries can discharge unequally
  • there will be a large current spike when two batteries are paralleled (as the voltage equalizes), and if those batteries are significantly dissimilar, the large current could persist for a while, generating heat in the batteries and leads, as well as being an inefficient way to charge the lesser battery and a damaging drain on the greater battery

Wire in two sb50 connections on the robot with longer cables. When the voltage starts to get low plug in a fully charged battery, disconnect the discharged battery. Longer cables can make it possible swap into the robot.

I see no reason why wiring two batteries in parallel would be unsafe. I’ve made a number of “power squids” to connect two or even 3 batteries in parallel for our robots before. Just pair up the wires with one SB50 on one end and multiple on the other. We usually run a few bots in the local Christmas parade (about a 2 mile route, 30+ minutes) and have never had any issues. Just take the normal precautions about not shorting anything, not having exposed wires, etc. Just using your normal batteries in parallel will be a lot easier than some sort of marine battery. Although from my limited knowledge in batteries, any deep cycle 12V battery should power a robot fine too.

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Some people say this is always 100% super sketchy…meh.

Make the swap quickly and go on with your life. There are some circuits you could construct to limit harm here, but for, say, 5-10 swaps on batteries your were going to junk soon anyway, send it with moderate caution.

Chiming in to agree that 2 batteries in parallel isn’t inherently unsafe. (Ditto for two in series–matter of fact there’s wiring tricks to get battery connections to switch from series for operation to parallel for charging.) Or more than 2, even. The trick is to try to match the batteries as much as possible–ideally, same batch, same usage amount, and charged together (charging in the parallel rig is often the best option here).

Of course, that’s doing things the “right way” as opposed to the “make it work” way.

Batteries in series… also doable. Doubles the voltage, so don’t try it with 12V FRC batteries using typical FRC electronics, 90%+ chance of frying something. Charging can be a pain in series, though.

I used to work on large uninterruptible power supplies (UPS) for data centers and medical, commercial and industrial applications. The largest battery cabinets we made were the size of a large household refrigerator. They were full of the 7AH version of the batteries used in FRC, the same as used in home burglar alarms and the small UPS’ that one can purchase at Best Buy. The several hundred batteries were connected in series parallel to get the system voltage required.

Batteries were always replaced regularly (3 years) as a set.

The positive and negative connections were taken off opposite ends of a bunch connected in parallel.

The automotive department Walmarts that have them sell deep-discharge marine batteries relatively inexpensively. They are available in different sizes and capacities. The one I purchased for my truck camper lasted for quite a few years, under the hood of the truck, in the southern heat, until I sold the truck.

I would consider three options:

  1. Get a lithium battery capable to doing the whole parade in one shot. Won’t be cheap, but also won’t spill/leak.

  2. Wire two battery leads to the robot, i.e. two batteries in parallel, with a couple power diodes. With this you can install a new battery and remove the old one without power cycling the robot. The power diodes are there to ensure the new battery doesn’t try to charge up the old battery for the few moments they’re connected together.

  3. Put the radio and maybe the Rio on a separate 12v battery backup so that changing main batteries doesn’t incur the 45s penalty of radio boot time.

We do this without the diode and haven’t noticed any issues.

I’ve looked into buy an Optima blue top to use as a parade bot battery and seemed like it would be a good option.

This is penty safe with a little care.

Alternatively, you could ditch the Rio-radio setup and go with an RC-conversion. This does only work with certain robots, and you may not get all the functions of the robot. Swerve is do-able, but may not be practical, pnuematics just need relays or an old motor controller. I found that multiple motors can be wired to a single channel.

The diode would be there in case disparate charge level batteries were left connected by accident. When the batteries are connected like this they would try to equalize charges and pass on the order of 50-150A between each other. The discharging battery wouldn’t care too much, but the battery that is charging (at over 2.5C) is likely to dissociate water in the electrolyte into oxygen and hydrogen, causing permanent damage. If you’re diligent about swapping quickly (a few seconds) this is not a problem. If they were left connected for longer than momentary then you start to have issues.

This sounds like more work than a battery hot-swap setup and more expensive than a lithium ion battery.

My main worry would be differences in voltage and rapid charging/discharging of batteries. I don’t know much about SLA batteries but I have caused too many Li-ion batteries to catch fire.

I agree. If done properly, it would be safe. I don’t know what “properly” means, and what resources it requires.