Battery question - series config of SLAs of the same capacity but different voltages

[Slats]

This ones doing my head in.. I'm trying to find a definitive answer but the noise of the internet is clouding much of the signal.

I have a large target project I'm working on. It requires 18v to get the 24v Bulher Motors working to thrust her up to a good clip.

Ok the question and this relates to SLA chemistry. I know its not a good idea to mix batteries of different capacities (Ah) together in series but if the capacities are the same, and the voltages are different is this ok?

I.e.e.g 12v SLA 7.2AH in series with a 6v SLA 7.2AH. This pumps out 18v nicely, but is there a problem with this?

I could go and get 3 x 6v SLA 7.2AH in series for the same 18V result, but I have a the 12v 7.2AH units and might as well use them.

A lot of the advice on the internet says its best to series connect batteries of the same capacity, chemistry, and voltage. I get the reasoning for the first 2 conditions (capacity and chemistry) but not the third voltage. Moreover, I understand the cell make up of SLAs is that they are all made up of 2v cells internally in series anyway. Meaning that a 12v SLA is 6 x 2v cells in series, a 6v SLA is 3 x 2v cells in series. If this is the case, then external series wiring of a 12v SLA and 6v SLA of the same capacity is surely then the same as 9 x 2v cells?

Andy and others please jump in.

Thanks

John

[KevinMc]

Hi John,

The only reason I can think of not to have the same voltage battery in a series connection is that perhaps the internal structure of the batteries are just different enough that there's a different efficiency during discharge so you'd perpetually wind up with an "out of balance" pack. Were you going to charge them up as an 18V cell, or as 6V and 12V packs separately? In the case of the latter I can see no issue at all. In the case of the former, it's something to keep an eye on, but I don't think it's an unreasonable configuration.

[Slats]

Hi John,

The only reason I can think of not to have the same voltage battery in a series connection is that perhaps the internal structure of the batteries are just different enough that there's a different efficiency during discharge so you'd perpetually wind up with an "out of balance" pack. Were you going to charge them up as an 18V cell, or as 6V and 12V packs separately? In the case of the latter I can see no issue at all. In the case of the former, it's something to keep an eye on, but I don't think it's an unreasonable configuration.

Thanks Kevin
the two SLA batteries would be charged separately as 12v and 6v on their own, and on their own 12v and 6v chargers. They are only joined by a jumper to make the 18v series connection in the boat.

Thanks

John

[roedj]

John,

I too see no immediate problem with this arrangement if they are charged separately.

But, the current capacity of this wiring scheme will be no greater than the current capacity of the weakest battery if I understand Kirchoff's Laws correctly.

Dan

[HvyCGN9]

how much space will you have in said hull??

I would run two 12v7.2ah batteries for 24v and turn the endpoints on throttle down...or...assuming you are using electronize(which can handle 24v) esc's turn the esc down abit with the adjustment screw.

Cheers Bruce

[Slats]

John,

I too see no immediate problem with this arrangement if they are charged separately.

But, the current capacity of this wiring scheme will be no greater than the current capacity of the weakest battery if I understand Kirchoff's Laws correctly.

Dan

Thanks Dan. Maximum current draw will be 3 amps.

J

[Slats]

how much space will you have in said hull??

I would run two 12v7.2ah batteries for 24v and turn the endpoints on throttle down...or...assuming you are using electronize(which can handle 24v) esc's turn the esc down abit with the adjustment screw.

Cheers Bruce

Hi Bruce
thanks anyway but already tried that but the Anzac won't take this weight - too heavy with that configuration, she goes beyond the water line.

I hope to have her running and / or close to complete by the time of the Regatta in Naracootte.

Hope to see you there.

J

[HvyCGN9]

Ah!!

I had same issue with the ANZAC I am slooowly rebuilding for Graham, needed a bit more grunt for scale speed and tried a few combo's of batteries. Everything from two 12v 7.2ah, then 3 6v10ah (which proved to be too bow heavy...), finally settled for a pair of 7.2v6.8ah Nimh stick packs ($25 ea from ebay with aussie type 240v trickle charger) in series giving 14.4v which gives a good speed and reasonable runtime....


HMAS ANZAC with Scott's HMAS Parramatta last year....Parra is a little faster she runs Mazda fan motors($5ea from auto wreckers....)

Cheers Bruce

[Slats]

Ah!!

I had same issue with the ANZAC I am slooowly rebuilding for Graham, needed a bit more grunt for scale speed and tried a few combo's of batteries. Everything from two 12v 7.2ah, then 3 6v10ah (which proved to be too bow heavy...), finally settled for a pair of 7.2v6.8ah Nimh stick packs ($25 ea from ebay with aussie type 240v trickle charger) in series giving 14.4v which gives a good speed and reasonable runtime....

[ATTACH=CONFIG]27959[/ATTACH][ATTACH=CONFIG]27960[/ATTACH][ATTACH=CONFIG]27961[/ATTACH]
HMAS ANZAC with Scott's HMAS Parramatta last year....Parra is a little faster she runs Mazda fan motors($5ea from auto wreckers....)

Cheers Bruce

Yes Anzacs seem to be a bit of interest proposition to get right. By the way do your props inward turn towards the rudder or outward?
Cheers

John

[HvyCGN9]

ANZAC & Parramatta's are both setup to outward away from rudder, Scott had Parra setup with them turning inward (as on the real ship) and whenever she turn would get alot of cavitation and gurgling. Swapping the props over she ran way nicer....

Cheers Bruce

PS one of the new guys here was telling me about some hefty 10ah Nimh D sized (with soldering lug) cells you can get to make up your own packs (ie 9.6v 10ah packx2...for 19.2v) way lighter than an 12v SLA

[KevinMc]

So if the right combination of weight/voltage/capacity is proving difficult to achieve why not go LiPo? (I'm sure you've got your reasons, I'm just curious to know what they are.)

[greenman407]

I think its because he already owns the batterys in question and doesnt want to buy anymore...........I Think.

[Slats]

So if the right combination of weight/voltage/capacity is proving difficult to achieve why not go LiPo? (I'm sure you've got your reasons, I'm just curious to know what they are.)

Kevin, its fundamentally what Mark "Greenman" noted. I have heaps of SLAs already..added to that the fact is we are talking about a model ship here that is 5.4' long and is a displacement hull. The hull will take some 4-5 kgs 11 pounds of weight to get her at the water line so SLAs double duty - provide power and ballast. Current drain is also nothing like what I see in most model boats that are designed for racing and speed. Also Lipos here create Nanny state issues for some elements that I sail with. If I departed from SLA technology for a surface ship, I'd go to NiMH, which incidentally I run in my subs.
J

[Slats]

ANZAC & Parramatta's are both setup to outward away from rudder, Scott had Parra setup with them turning inward (as on the real ship) and whenever she turn would get alot of cavitation and gurgling. Swapping the props over she ran way nicer....

Cheers Bruce

PS one of the new guys here was telling me about some hefty 10ah Nimh D sized (with soldering lug) cells you can get to make up your own packs (ie 9.6v 10ah packx2...for 19.2v) way lighter than an 12v SLA

Thanks Bruce - good info. I'm sort of considering the NiMH route, but I have so many SLAs here to use. The problem I see with these big NiMH packs is the charging time. I'm also a bit lost on where to find a good solid charger for NimH larger capacity packs, that comply with our good standards in Australia. I have seen Hobby King just put out a statement recently about "fake" chargers that don't comply.

Bruce let me know if you have a link to the cells and a charger that has with it and Aussie compliant 240v mains power supply.

Best

John

[HvyCGN9]

I realised earlier that I actually got the charger from a mob in Oz (I will add links to the charger and some ready made Nimh packs shortly)

Bruce

Battery charger: http://www.ebay.com.au/itm/390481007...84.m1423.l2649

7.2v - 12v Nimh packs :http://stores.ebay.com.au/bestpriceh...4.l1513&_pgn=1