Be careful with the cheapie. Non "smart" trickle chargers can still cook a battery. Check the specs/instructions on the Battery Tender to see if you can charge two batteries at once... in PARALLEL, NOT series. I can do this with my Battery Minder's, but they say to charge each battery fully first, then combine.

Best thing you can do for a battery is to keep it on a smart charger all the time. In lieu of that, charge fully and then remove the negative battery cable.

Be sure to check your fluid level and only use distilled water.

Are you talking about the Battery Tender by brand name? I seem to recall that Yamaha recommends the use of them.

The Battery Tender Jr. only outputs .75 amps. Nothing to worry about from an over charging standpoint with such minimal current flow.

As Dennis says, the batteries will be connected in parallel, not in series. Unless you are using a 24 volt charger that is.

If a battery is in good condition, it will not require charging for quite some time, however, if you must charge them, do them one at a time, multiple charging can be done, and often is, but far better to leave one on "trickle" then swap to the other battery.

Why is this?

Will one battery somehow know that another battery is connected to it in parallel? How will it know?

I have looked at several sources for "Why Batteries Fail"

Here's my 20 cents (inflation)!

Below is an excerpt from https://www.emarineinc.com/Batteries-Maintenance-101
"When a lead-acid battery is discharged, a soft lead sulfate material forms on the battery plates. During the battery's recharge, this material is lifted off the plates and recombined into the battery's electrolyte solution. If, however, the battery is left in a partial state of discharge for as short as 3 days, the lead sulfate material will begin to harden and crystallize, forming a permanent insulating barrier. As this barrier becomes thicker and thicker, the battery's ability to accept a charge or deliver energy is diminished, resulting in the perception that the battery is no longer usable. The accumulation of such deposits, otherwise known as sulfation, is the most destructive process in the life of any lead-acid battery."

IMO, best to do when storing a battery for extended length of time, make sure battery is fully charged before storing, maintain charge with auto charging charger (or trickle charger, if that is only what you have). Before reconnecting battery, perform "equalization" procedure as in the link and per interval recommended.

I invested in an onboard charger that takes care of all what I've said automatically. https://www.westmarine.com/buy/profe...ecs-_-MB-_-PDP

Before I invested in this, I have pulled my batteries and had them load tested where I bought the batteries from. I thought my batteries were ok, but to my surprise, they failed the CCA/MCA test. Barley, but still failed. They were under warranty and the supplier replaced them under warranty. I'll do this each season to ensure my batteries are ok.

These batteries are new about 1-1/2 years ago, I expect to get 3 or more years out of them.

Be careful buying a battery tender from stores like Harbor Freight. I had one and thought I'd have a fully charged battery when spring came around...wrong.
The tender failed and fried my battery.

The onboard charger is the way to go. Buy one with lights that indicates if the battery is charging or fully charged.

I installed one on each of my boat but only plug them in the night before I go out. But then again, since I've retired I normally boat year round now.

Good Information

I have been using battery maintainers on my RV and boats with great success. SEM-1562A - Schumacher Electric . With lead acid batteries and best result for longevity, don't let a 12 volt battery discharge below 12.0 volts DC. Sulfanation of lead acid batteries and pounding on waves and buckled plates from both complete dicharge and over-charge(shorted cells) are destructive.

I reviewed the manual for that US 4.3, looks like a good compact unit Scott! Have you ever used the "Recondition mode"? Looks like you have to place in that mode manually to do so?

One of the reasons I like the Battery Minder brand is for the reconditioning modes mentioned above in similar units. It's a nice, compact unit (like a Battery Tender) but it adds a (automatic) desulphation mode. I probably have about 10 or 12 of them - some of them for 10 years, now. There a lot's of good options. In my Sundancer I have a ProMariner onboard charger, but it's the bigger unit - the ProNautic series. Great charger.

Could it be that some people have issues with THe Battery tender jr or similar automatic chargers/maintainers because they use it as a charger as in my case in using it to just maintain it for 3-4 months ?

Although it would take much longer, the small ones (some brands DO de-sulphate) will still charge a battery if it's been worn down. Slow charging (essential towards the end) is actually the best thing for a battery to give it a FULL charge. However, not all maintainers are created equal so it's important to buy based on quality/brand/features, not just price. While a "cheapie" brand might say that it's a maintainer... what if it's not very reliable?

Think of charging like filling a glass with water. You can start out filling it full blast from the faucet, but as the level rises, you need to slow down the flow rate and eventually to a "trickle". Then it get's monitored and as the water evaporates from the glass, a few drops are added as needed to keep it full. If the flow stays on (even a trickle) it can still overflow the glass (cook a battery).

Hands down the best way to get the most from a battery is keeping it charged. I use a solar panel with an IC chip that regulates voltage.

Yes, generally not good to charge in parallel. The weaker one will draw from the stronger. I use diodes with the solar panel to prevent this.

The converse is don't let it discharge. For that I use a perko two battery disconnect switch that eliminates parasitic loads when stored.

I don't use the boat near as I'd like, but with this setup the batteries that came with the boat in 2010 are still in service.

BTW - great info here on de-sulphate.

I have had cheap smart chargers that fail.
Maybe it is that I use them to charge a flat battery, demanding a large current and then expect them to accurately slowly top up and not overcharge. They may suffer from poor regulation this way from that heat from high initial current. It may be that they should be only left on float on a battery that is almost full.
Maybe that is why there are small output chargers out there that should only be used in a situation where the battery's immediate demand state matches the chargers output ability.
I also worry about anything that is attached to the mains all the time. Eventually a lot of things do expire. E.g. Chargers for phones, notebooks, WiFi etc.

I too have a couple CTecs 5 amp Australian models that I am hoping will last a long time - albeit not willing to leave on a battery for more than a week . Maybe in time I'll get over that fear.

It would also seem that you get what you pay, hopefully these costly little things (for their Amperage output) standup to time.