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[ray t]

I am looking to run two 550/1 MFA motors on my cat.  Now I have decided to run two 7.2v packs in parrallel to increase my run times as the extra weight will not be a problem.  In fact, I am going to run a third 7.2v pack for an emergency backup maybe.

My question is, do I simply hook the two batt packs up in parrallel then run the output straight to the P94 (that I am still to buy).  Then from the P94 I will have two seperate outputs that will go to my two motors?

[Shipmate60]

In a word YES.

Bob

[ray t]

Come on Bob, 2mins 15secs I had to wait for that reply 

Thank you as always sir

[FullLeatherJacket]

In another word, NO. You run the risk of one pack discharging into the other with potentially dangerous results. You must also fuse the connections to the P94 as per the instructions which accompany it.

As and when you order the P94 we can discuss the options available, including providing for a third pack as back-up. If you're impatient then check out P103 Parallel Power Board on the ACTion website.

FLJ
(Sorry, Bob!)

[Shipmate60]

Dave,
As you know as you make them the P94 has 1 power connection 2 outlets.
I think we will always disagree on paralleling batteries.


Bob

[DickyD]

Question - What is the difference between 6 C cells in series and 12 C cells in series, and 6 C cells in a battery pack connected in parrallel to a further 6 C cells in a battery pack ?

[Shipmate60]

The connections are different!!!       

Bob

[DickyD]

Sensible answer for a numpty Bob, I run several of mine on parrallel battery packs.

[FullLeatherJacket]

Question - What is the difference between 6 C cells in series and 12 C cells in series, and 6 C cells in a battery pack connected in parrallel to a further 6 C cells in a battery pack ?

6 in series =7.2v
12 in series =14.4v
6+6 in parallel =7.2v (and a D for your spelling).
I appreciate that you've both managed your battery packs correctly in the past, but that doesn't mean that everyone else will do likewise. You need to put yourself on this side of the counter for seven days to see what happens in the world outside your workshop 
FLJ

[Shipmate60]

No thanks Dave, just happy to test them for you when built.

Suits me.

Bob

[Prophet]

Try the actions P103 parallell board for a quick setup that you can plug to your p94 extra £14 but worth it

http://www.action-electronics.co.uk/pat.php

[DickyD]

Try the actions P103 parallell board for a quick setup that you can plug to your p94 extra £14 but worth it

http://www.action-electronics.co.uk/pat.php

And me a founder of member of Daves fan club. Grandmother and sucking eggs springs to mind.

As for the spelling Dave, the spell chucker didnt pick it up. It must be a Yank.

[malcolmfrary]

The problem when connecting batteries in parallel is making sure that they are both equally (very equally) charged.  Any difference in voltage can result in very large currents while they sort themselves out.  If theres a dud cell in one of the packs, it will result in whatever that dudness was reflecting in the other pack, either by the entire load being shifted to the good pack, or the reduced voltage in the dud pack resulting in the good one trying to charge it.

[Colin Bishop]

If you have two motors then there is nothing to prevent you having a separate battery pack and speed controller for each one in which case there is no 'crossover' electrically. You would of course have to charge the battery packs separately though.

Colin

[RaaArtyGunner]

Now that I am utterly confused,  I take it we talking battery charging in general and Ni cad's, niMh's and hobby use in particular ie boating.
In motor vehicles, batteries are often connected in series or parallel ie trucks.
However is there then some device and wiring unseen to us laymen that guards against the dead cells etc.

[Colin Bishop]

There is an assumption made that cells grouped together in a single 'box' such as an SLA battery constitute a single unique battery which should be charged independently. To a certain extent this is true but if you were to purchase two identical 6 volt SLA batteries and connected them in series then you could reasonably treat them as a single 12v battery. If they were not identical with one having a bigger amp hour capacity than the other then this would not be the case and you would be in trouble.

When it comes to NiMH cells then you make can make up a pack using any number of identical cells. If you were to add a further pack in series, using identical cells, then you could still treat the whole assembly as one battery and charge on the basis of the total voltage.

The problems occur if the component cells are not identical either in terms of capacity or in age in which case you can get a charging imbalance. This can of course also occur in a 'proper' battery if one of the internal cells fails as they sometimes do.

Therefore, the most prudent course of action if using lead acid is to fit a single 'battery' of the required capacity and voltage. If using NiMH then make up a battery of the required number of identical cells by purchasing them all at the same time. This will not guarantee failure of single component cells but will significantly reduce the risk.

Not rocket science, just a bit of common sense and understanding.

Colin

[RaaArtyGunner]

Colin, Thank you.
Therefore, am I correct, the risk is with paralleling batteries but not when in series.

[Shipmate60]

Yes,
Before paralleling batteries of the same capacity and age I always check the voltage of each pack after charging individually.
You can charge as Colin says, but I just like another check before connecting them.
If a cell goes down it will show on each pack voltage, if it goes down under load you wont know till they are checked before charging.

Bob

[Colin Bishop]

The risk is that, however you connect up your batteries, if the characteristics are not identical you can have problems. The safest options with twin motors are either to have one big battery supplying both motors or two entirely separate battery/motor configurations. The first option is preferable as you only have one battery to charge. If you try and charge two batteries in parallel then that is the most lilkely situation when problems will occur. Full size yachts often have parallel battery setups, sometimes with varying capacities, and the charging arrangements require clever electronics and diodes to ensure that each battery gets the correct charge. Our models don't have that sort of setup!

As in all things the KISS principle applies (Keep It Simple Stupid)

Colin

[RaaArtyGunner]

As in all things the KISS principle applies (Keep It Simple Stupid) 

Colin

And really that is it in a nutshell. 
Many thanks . 

[RaaArtyGunner]

As an aside, is there anyway of finding out which cell is dead in a pack without destroying the pack.

Most packs seem to be spot welded, which I suppose means you have to cut the connecting strips to find the problem cell and then reconnect (solder?) the cells back up again.

[malcolmfrary]

The only non intrusive way I can think of would be to have a sensitive thermometer and run it along the pack when charging - the dud, depending on the mode of failure, would show a different temperature.
The other way is to use a pin as a meter probe, and poke through the insulation of the pack, noting the voltages cell by cell.  Not really recommended, since you wind up with a pack with a row of pinholes in the insulation.  If the application of the pin is over enthusiastic, there is also the risk of puncturing the cell being looked at, creating a further problem.  This only applies to NiCads and NiMH batteries, I wouldn't consider poking at Lithium, and since SLAs are effectively a sealed unit with no consumer serviceable parts.
Of course, when the dud has been found, the pack is in for either surgery or burial.

[RaaArtyGunner]

Thank you