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[dougal99]

Hi All,

I've just read Paul Freshney's and Paul Brumstead's column in the August 2006 MB where they refer to preferring to switch between two batteries as opposed to wiring them in parallel to obtain extended duration. However, there is no reference to why or what advantage is perceived. Can anyone shed any light on this subject and give opinion/experiences as to the benefits?

Arranging a switching system seems a lot more work than making a parallel wiring harness ???

Cheers

Doug

[DavieTait]

Hi Doug. If you wire batteries in Parallel you keep the same voltage but ADD to Amp-Hour of both batteries together , thus giving longer running time. If you wire batteries in series you keep the same Amp-Hours but add the voltage together , IE put 2 6V batteries in series to get 12V.

The idea of putting in a switching system seems to be adding yet another chunk of wiring into the system that can potentially go wrong , would need another channel on the Tx-Rx , and either an electronic switch activated off the Rx or a servo activated switch. There is no need at all to put in a switch as far as I can see. If the problem is one of sudden power loss ( ok with lead-acid you notice the power drop off gradually , BUT , with rechargables - especially NiCads - the power seems to drop off very quickly leaving you with no way of getting back to the pond side ) then I can see the point but if you time your runs you will start to get a handle on how long you can safely run and all you need do is make sure your back at the pond side with 10% time remaining in case of problems.

Davie

[sweeper]

The danger of parallel operation of batteries or cells is that unless the batteries are identical in charge state and condition, you can set up a constant circulating current between the batteries as the higher charge one attempts to charge the lower charge one. This will occur regardless of whether or not you are using the boat.
If all you want to achieve is a longer duration running time, the simple answer is to fit a larger capacity single battery.
My advice when dealing with things electrical is always revert to this kiss theory. It saves a whole lot of trouble.

[RickF]

In an ideal world, Sweeper is correct: the best way to increase battery capacity is to fit a bigger battery! Unfortunately our models don't operate in an ideal world, as factors such as accessibility, beam and balance play an important part. It is often necessary to spread considerable weight about the hull to achieve the correct balance/waterline and the easiest way of doing this is with lead/acid batteries.

The answer with parallel multi-battery setups is to use identical batteries, as far as possible, with isolation switches on each. You can then use them all, or individually in turn and isolate the lot when not in use. I agree with Davie that an R/C switching system would be a potential weak point, but unless you intend to break endurance records, bringing it back pondside to switch over the batteries should not present a problem.

Rick

[BobF]

Hi Doug,

You have not said what type of battery this refers to, ie nicad or gell cell etc. and I have not read the article.

But I do remember an artical from a few years ago, that ni cads should never be be connected in parallel, because if the cells in the battery are not identical, which is a naturally occuring thing during usage, it can cause individual cells to go into reverse polarity. Which is why the top fast electric racers, discharge each cell in the pack individually.

Having said this, I have noticed several semi fast electric cruiser kit suppliers, now recommend two sets of nicads in parallel. So ???

Regards Bob

[wombat]

I would be cautious about connecting batteries in parallel - if they are in different charge states, then one will try to charge the other causing a circulating current which could be quite high, limited only be the internal resistance of the cells. This current could possibly damage one or more of the cells. The bigger problem IMHO is if one of the cells in the parallel arrangement fails - the other battry will try to charge the one with the failed cell, with a very high current. Without fuse protection it could cause the whole battery system to go into meltdown - a limited risk, only about 1% of battery failures for a VRLA are shorted cell.

My preference for paralleling batteries if it has to be done is to use a Schottky diode on each battery, which will prevent the circulating current especially if a cell fails.

Tim The Wombat

[flag-d]

Tim: please explain the schok shotkady skocty whatever diode thing.  Please use short words and speak slowly and assume I'm an unknowing buffoon!

Mike

[Shipmate60]

I have used 2 x NiCad packs in parallel for years without any great problems.
Not on high drain motors though.
I usually fast charge and trickle charge to top right up, then leave for about a day.
Couple the 2 batteries in parallel the night before sailing and so far havent had any problems with duration or charging.

Bob

[dougal99]

Hi All

Thanks for the input. It appears that general experience/knowledge is that apart from the possible problem of one battery trying to charge another (which I was aware of and should be minimised by using freshly charged batteries - who doesn't?  ) there is no real problem with using batteries in parallel. Which is good as I started (to be exact Dad started) in 1958 in a trawler built using a free plan in the Hobbies annual.

BobF

The batteries under discussion in the article were NIMH sub C packs 3000-3300 mAh.

Thanks again to all who contributed

Cheers

Doug

[wombat]

Tim: please explain the schok shotkady skocty whatever diode thing.  Please use short words and speak slowly and assume I'm an unknowing buffoon!

Mike

Hi Mike,

Well, a diode allows current to flow in one direction only - from anode to cathode (the terminal marked with a line on the actual component). So if you connect a diode withe the anode to the positive of each battery, connect the cathodes together. This will allow current to pass from each of the batteries to the circuit, but current cannot feed back from one battery to another.

The downside of this is that you drop voltage across the diode - you cannot eliminate this but Schottky diodes have a voltage drop of around 0.3 to 0.5V as opposed to 0.7 to 1V for a conventional diode.

If you do this, the batteries have to be charged separately, however if you use a constant current charger (not a smart charger, which rely on measuring the battery voltage) you can rig more diodes to allow the batteries to be charged together - connect the anodes together and connect to the charge socket, connect the cathodes to the positive of the batteries - in this case current can flow from the charger to the batteries but cannot circulate around the batteries.

Hope that helps a bit. If you think of the diode as a one way valve, it might make it easier to understand.

Tim the Wombat

[BobF]

Hi flag-d

SCHOTTKY Diodes should be used when using a forwards only electronic speed controller, and are fitted between the motor terminals.

I believe they stop a reverse current being sent back into the controller and destroying it. It  also says that it makes the controller run cooler.
I did once connect one up withought a Diode and it never worked.

Bob

[malcolmfrary]

The nasty thing about paralleling batteries without the diode arrangement is that it doesn't matter if they are charged and discharged identically - in the nature of such things, they all have a finite life expectancy of charge/discharge cycles which will be similar, but not identical.  This means that in the fullness of time, one cell will fail before the others. 
If it fails by going high resistance, the only effect will be that the run time is shortened. 
If it fails by shorting out (even only partially), it will cause the massive circulating current as has been mentioned, which will probably be in a part of the wiring not protected by any fuse.  If the wiring and model survives this, then the symptom will be that the entire bank of batteries will fail to charge properly. 
The diodes or switches is preferable, but returning to the pondside to swap battery connections is probably simplest.
Like Sweeper said, KISS.