What's better... smaller & more or larger but fewer lipos?

[henrysj]

Trying to work out the best way to go... either 8500 or 10000 mah with 4 in parallel or 16000 or 22000 mah with 2 in parallel

Ive done an ecalc on each using Tattu 10000, 16000 & 22000mah as well as the Vulcan 8500mah superlight.

I'm not totally convinced what ecalc is telling me - I'm not fussed on the actual flight times it's giving me just the difference in times it thinks between each setup. With the same total battery weight & mah it thinks 2 in parallel compared to the equivalent setup with 4 in parallel will require more throttle & more flight time?

If anyone could shed some light on this and which is the best setup that would be greatly appreciated.

 

[SleepyC]

From what I can see, if your rig is that heavy MORE would be better. (2) batteries can only supply so much power at one time. (4) can supply that power easier.

Think of it as people trying to go through a tunnel. If you have a going and coming tunnel that each side is 6 feet in diameter and you need to get 200 people through that tunnel and back it'g going to be cramped, hot and probably some undue resistance will take place. BUT if you have (3) sets of tunnels next to each other that are also 6 feet in diameter and send the same 200 people through all three tunnels, there will be less resistance, everyone will be cooler and the system will be generally more efficient.

 

[Bartman]

ummmmmm......ok

generally speaking it comes down to how many amps your helicopter needs to run and how many amps the batteries can provide.

1 battery than can provide 120 amps continuously should do about as well as 3 batteries that can each provide 40 amps or four batteries that can each provide 30 amps. how much one or three or four weighs does matter and will contribute to a best solution but if ecalc is saying they all weigh the same then there shouldn't be much of a performance difference between each scenario unless discharge ratings are insufficient making any one combination inferior to the others.

there is also the question of redundancy.....can two batteries help keep the helicopter flying if one cell fails or if a battery connection comes loose and to expand on that can four batteries provide even more protection against failures? I believe in building redundancy into my helicopters so I almost always will use at least two batteries and the frames I fly actually have room for four.

another thing to consider is that if a really big battery chunks a cell then you're out A LOT of money! smaller batteries will still cost you some scratch but it's less so maybe four helps the bottom line in the long run? have you also considered MaxAmps packs? they appear to be smaller/lighter than some of the other high MAH brands.

 

[Motopreserve]

Are you guys running 2 separate leads to the PDB when you run 2 batteries? When I first set up for dual batteries I had soldered up a Y-squid. Then I realized that this could be a bottleneck (2 batteries funneling into a single wire pair).

Ever since then, I've run 2 different pairs/leads to the PDB - so each battery has its own "tunnel" to the distribution. Not sure if it's necessary - but it made me feel better at the time. And now I just keep with the same system.

 

[Bartman]

two sets of wires to two different sets of contacts can help improve survivability when using multiple batteries but a Y will perform just as well if the wires are the correct size for the current being drawn.

 

[Motopreserve]

Good to know. I like the idea of survivability - I think I'll stick with that

 

[SleepyC]

two sets of wires to two different sets of contacts can help improve survivability when using multiple batteries but a Y will perform just as well if the wires are the correct size for the current being drawn.

Well that is where my analogy came from.
If you Y something, how do you factor in the added resistance that can and usually does add heat. Even the best soldier joint adds resistance.

From real world experiences, when you add direct leads to the PDB and you draw from all 3 batts instead of just 2 the entire system runs cooler. In fact, I recently changed a rig that had big 8G wires coming out and then Y'ed into some 10G wires to grab power from 2 6S packs. I redid the power system and added two separate leads into the PDB 10G. The entire power system ran 15 - 20 deg cooler. The cables were noticeably cooler to the touch.

 

[Motopreserve]

I've always erred on the side of caution with the AWG, but good to hear that you saw improvement in an actual application.

Thanks folks.

 

[eskil23]

For a small craft I would say that fewer batteries are better because of less weight. But when it comes to this size I would say the opposite; The weight saving would be minimal with such large batteries. Amp throughput is probably more important.

 

[henrysj]

From what I can see, if your rig is that heavy MORE would be better. (2) batteries can only supply so much power at one time. (4) can supply that power easier.

Ah this explains why ecalc gives much better performance for 4 packs compared with 2 - hadn't considered this this before

My setup has 4 battery cables soldered direct to the power board so the benefits of having 4 packs are ready for the taking. I guess another slight bonus would be the weight of the batteries would be more evenly distributed.

I was concerned that the charge time would be longer as I've never had the need to parallel charge before with a duo charger but see that makes no difference.

so seems like multiple packs it's gonna be - cheers for all the advise

 

[violetwolf]

Great discussion. I agree with all posters and will add that with more/smaller batts you can use them on your smaller beater drones too

 

[Pumpkinguy]

Choose your pdb wisely. A typical board with one or two layers of 2 Oz copper for a rig like this scares the crap out of me. Solid copper would be better. 2-3mm thick. Solder bullets to it or wires directly with bullets on them.

 

[SamaraMedia]

My Tarot X6 has a built in pdb, I'm not a fan of it but that's what you get with a kit like this. It comes with one XT150 or AS150 (not sure of the difference at this hour) but I'm made an extension cable with EC5's for ease of use and finding them locally. I just bought a couple SLS7000 6s batteries to replace some beat SkyLipo 5000 6s I've been using. They look pretty nice, compact size and weight, measure about the same size as the 5000's and weigh just 100g's more each so I think the weight to power ratio is acceptable. I made a pigtail from 10AGW to run two in parallel, same gauge wire on the leads coming off the pdb. Hopefully it will be an improvement from what I have now.

Now to get back to tuning my SuperX on this rig.

 

[Motopreserve]

Choose your pdb wisely. A typical board with one or two layers of 2 Oz copper for a rig like this scares the crap out of me. Solid copper would be better. 2-3mm thick. Solder bullets to it or wires directly with bullets on them.

That's something that hasn't been mentioned yet. But the PDB can definitely be a weak link - and then all this effort for quality wiring, connectors and expensive batteries can all go down the drain quick.

 

[Pumpkinguy]

35 pound rig. If there was ever an excuse to use something like a power hungry board...... http://powerhungrysystems.com/?page_id=21

Expensive I know but how much is peace of mind worth?

I made my own version of the power hungry here:

http://multirotorforums.com/threads/gryphon-1000-hex-build-with-ecalc-data.21951/page-3

 

[henrysj]

So little update...

Noticed a quite a difference between running 4 packs and 2 packs - with 2 packs I'm getting much bigger voltage drop under load and 4 packs are definitely keeping the lipos cooler - so many thanks for the advise and saving me getting some 22000mah packs.

Flight times with rig weight of 8.12kg, payload of 6kg and batteries listed below, landing when first warning comes @ 21.6v :

2 x 16ooo mah packs (3.9kg) - 6.40min - on landing battery has 37% left

4 x 8500 mah packs (4kg) - 8min - 21% left

4 x 10000 mah packs (5.2kg) - 9.30min 16% left

Using a PDB board rated to 400A

 

[Bartman]

henry,

a big contributor to performance and ability to sustain voltage levels under load is the Discharge C Rating. Batteries have a Charging C Rating and a Discharge C Rating. What are the Discharge C Ratings of the different packs you're comparing?

 

[henrysj]

Hey Bartman,

Tattu 16000mah are 15C to 30C Burst
Gens Ace 10000mah are 25C
Overlander 8500mah are 15C to 30C Burst

I'm guessing just having 2 packs at 15C isn't sufficient, to be honest I'm not sure how you calculate this though?

 

[Motopreserve]

Sorry to jump in, I know this was directed to @Bartman.

To calculate the discharge capability of the battery you just multiply the discharge rate by the capacity.

So in the case of the 16000mah Tattu above: (16A)*15C = 240A continuous.

The discharge rate does not increase by running them in parallel. Only the capacity increases.

 

[eskil23]

The discharge rate does not increase by running them in parallel. Only the capacity increases.

That is wrong. If each battery can output 240 A then two batteries can supply 480 A.