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Charging two batteries on one charger

13K views 17 replies 10 participants last post by  mitiempo 
#1 ·
If I start with both batteries topped-up, is it a problem to conntect two different size batteries to a charger in parallel? This is for winter storage.
 
#3 ·
I routinely connect two completely different batteries together to charge (both 12volt, one lawn tractor and one marine). I NEVER leave them connected together for longer than I am charging them, and usually for durations around 24 hours. If I'm pushing 10amps into both, then I don't worry about one draining the other, then it's just a matter of pulling the charger and the jumper wires.
 
#4 ·
Charge them both to full, independently, then stick them in your un-heated garage or shed etc.. By spring, in WI, they will still be damn near full. Cold weather, as in winter temps in the Northern US, DRASTICALLY slows self discharge and sulfation. If you want, or feel the need, top them up once or twice. No need to leave them connected and cooking away when not necessary. I've seen far to many charger failures that took out banks to even consider it for my own batteries.

Our boat sits next to the house and the batteries are still left on-board, purposely, all winter, with no charger or solar. If I work on the boat I may re-connect the bank and fire up the charger but that's it...
 
#6 · (Edited)
A lot of sailboats have deep cycle batteries and an engine battery. Both get charged at the same time when the engine is running. While at the dock, I bet most people will charge deep cycle and engine batteries at the same time, and float them once charge is complete with the same charger. Have there been any problems with this method? No one has said so as far as I know. I have read that an AGM battery can be overcharged with an ordinary charger as the voltage can be too high for them.
 
#8 · (Edited)
But when these batteries are charged on a boat they are often done so via a proper marine type charger with separate bank 1 & 2 leads. These chargers often use blocking diodes in the unit so the banks are really not in parallel.

Other methods such as switching a 1/2/BOTH to BOTH are usually temporary while "charging" and while on-board.. I don't like unattended use of BOTH to charge either but people do it. An ACR type relay only combines during charging and an Echo type device does not combine nor would an isolator. The ACR automatically disconnects the bank though when charging disappears and is fully automatic in protecting differing banks from being in parallel when not charging..

As I read it the OP wanted to hard wire the two batts together then charge as one. Technically this is fine when charging, but, if the power goes out, charger fails, or freaks out, which they do, the batteries would still remain hard wired and combined even without charging present. I suspect he wanted to leave them this way for the entire winter. I replace enough failed and freaked out chargers that this is not an approach I would ever take, leaving batts on charge all winter unattended especially when it is not necessary. Most boat yards specifically ban unattended battery charging during the winter months, all with good reason..

When there is no need to do so why hard wire two batteries together of differing ages, sizes or types.... ?

Also some manufacturers, Lifeline is one, specifically advise against leaving the batteries on constant trickle charge.

But of course you don't need to take my word for it Ed Sherman the ABYC's Curriculum Director just posted the quote below on Nov 10, 2011. Gota love it he's more direct and to the point about this subject than I am...:)

Ed Sherman - ABYC Curriculum Director said:
Let me begin by emphasizing one key point, there is no need or even a desire to remove the batteries from your boat! The habit some old timers got into with this procedure is right up there with setting a bettery on a concrete floor and the concrete sucking the life out of a battery like some sort of vampire. The concrete isn't the culprit and never was, and the need to remove batteries from a boat when it goes into storage for the winter is in the same mis-imformed category.

What is important is that the batteries get a full charge before you lay them up for the winter.
 
#11 · (Edited)
If you like that and are comfortable, then it is fine, but PLEASE do check on them frequently! I have seen far to many failures of chargers, all types and brands, to leave batteries unattended and on smart of "float" chargers. I simply won't do it to my own batteries and don't and still get above average life out of them. In the cold North we simply don't need to leave them constantly plugged in if topped up and disconnected.

In hot climates it can be a necessity due to self discharge as this accelerates in warmer temps and drastically slows in cold temps..

Most all boat yards up here specifically ban this practice in the winter, if batts are left on-board, and also stipulate that the batts must be left 100% disconnected from the vessel.

Some battery manufacturers also specifically advise against it, such as Lifeline.

Not all chargers behave the same or charge the same way. Some that claim float or smart provide a constant current and some pulse it ON/OFF based on voltage. The higher the float voltage setting the more often the current will be flowing in a pulsed charger..

If battery chargers were more reliable I may not have such an issue with it but I just see far to many failures and the resulting destroyed battery banks. With cold stored batteries I see leaving them plugged in and unattended akin to fogging your cars engine every time you shut it off. Not really necessary..

The other side is we have many unscrupulous manufacturers claiming "smart charging" selling pure unadulterated JUNK that is nothing of the "smart" sort...

This charger ruined a customers $1400.00 bank of NEW gel batteries charging them at over 16V on the GEL setting.... Smart? Hardly.??

Schumacher Ship n Shore Over Charging - YouTube

In a perfect world a proper float algorithm is fine but we just don't live in a perfect world. Battery chargers remain one of the most unreliable devices I see on boats. Please use them carefully...
 
#12 ·
Thanks for the replies everyone.

Wow, the cold Wisconsin winter could actually save me some work? Cool. Next year, I will just make sure the batts are topped off, and leave them on the boat.

I have a pretty simple electrical system, with a house battery and a starter battery. Where would I look to see if there is a diode protecting back current?
 
#13 · (Edited)
The above discussion worries me a bit. Not because some of the statements made are wrong, but because of the danger of their being misinterpreted.

I hope that no one takes away from this discussion the thought that it is always OK to charge your batteries before leaving them to themselves for the winter. While that might be a viable strategy in the cold frozen north, it is most definitely NOT a good idea for those habiting more temperate latitudes, i.e., those in which the temperatures during "winter" vary widely and/or stay relatively warm.

Keep in mind that all batteries have a self-discharge rate. Flooded batteries lose more each day/week/month from self-discharge than do, e.g., AGMs or gels, and this rate depends both on temperature and on age/condition of the battery.

If the ambient temp is very low, like near freezing, the self-discharge rate will be quite low. This is the condition which I believe MaineSail was referring to.

However, in areas like the Chesapeake daily temps during winter can vary widely....we even get some 70-degree days during December and January....and we can get some very cold (under zero degrees F) days and nights. If your boat is unheated the ambient temp will be affected by the outside temp and the amount of sunlight, as well as the degree of insulation.

What's the danger of leaving a battery less than fully charged? It can and will sulfate, and it will suffer from stratification (i.e., differing electrolyte concentration at various levels). These are the big killers of batteries which, unchecked, will result in loss of capacity and reduced overall life.

Remember, too, that even at a float voltage of 13.2-13.6, a flooded battery will sulfate and stratify somewhat. To avoid these conditions it is necessary to periodically increase the charge voltage to 14.4-14.8 or so.

If you're living in southern latitudes, say South Carolina to Florida or the Caribbean, don't even think about leaving your flooded batteries without at least some means to charge them periodically. In these climates, unattended flooded batteries can be killed by neglect in very short order.

Bill
 
#18 ·
With some chargers like the Xantrex (not the best but the most prolific I think) you can switch from 3 stage to 2 stage charging. After a full charge is reached the charger sleeps until the voltage drops to 12.5 and it then comes on to fully charge before sleeping again. It refreshes the batteries each 21 days automatically in this mode. This is better for the batteries than a constant float and takes care of any self discharge issues.
 
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