Higher voltage versus capacity for Nimh packs (limited space)

[theTman]

I just pulled my old crawler out of the closet after being away from crawling for 6 years. It's an old TLT axle based build with a gopher tube chassis, 55T motor, and a Mamba Max ESC (not the pro).

I found that due to my forgetfulness my old packs went dead due to not getting a maintenance charging to keep them alive throughout the years.


I need new battery packs and cannot afford Lipo right now. I don't plan on trying to compete with this old rig, but rather just have fun crawling. I can't afford a new rig either.

After doing some measuring I have found that I could increase the capacity of my packs or increase their voltage. I was running 7.2 volt 1500 Mah 2/3 A cell 2x3 brick packs. I have now discovered that I could either get a pack with those same cells but with 2 more cells, or upgrade to 4/5 A 2100 Mah cells but stay at 7.2 volts.

I know that higher voltage means more power and higher speed, but since I don't typically run at top speed I wondered if the battery might last longer than a 7.2 volt due to running at a power drain further from the max when compared to 7.2 volts. I also wonder if that would be better, worse, or about the same as increasing the capacity by just 600 mah. I also wonder if changing the gearing when increasing the voltage could get me better runtime but still enough performance.

[djjiz]

Run time doesn't change with more or less voltage.


And get a lipo.

[WHITE-TRASH]

Originally Posted by djjiz

Run time doesn't change with more or less voltage.


And get a lipo.

Agreed.


Don't be a nihmrod. Small lipo's like you're after are CHEAP like $9 and less cheap for a 1300 Mah pack that'll last longer than a 2200 Mah Nihm. Go browse hobbypartz.com or hobbyking and you'll find cheap packs and a $30 or less charger.

[theTman]

Originally Posted by djjiz

Run time doesn't change with more or less voltage.


And get a lipo.

I kind of figured run time wouldn't change. I just had a thought that running at a lower percentage of the batteries full output versus a 7.2 volt might make it last longer. Like, if you run a 7.2 volt and only go about 70% top speed on average versus a 9.6 which increases top speed but you only run at maybe 50% top speed on average. I know that if you run full bore it doesn't matter.

I guess I could look into a Lipo. I was looking at spending about 60 bucks to get 3 packs.

Originally Posted by WHITE-TRASH

Agreed.


Don't be a nihmrod. Small lipo's like you're after are CHEAP like $9 and less cheap for a 1300 Mah pack that'll last longer than a 2200 Mah Nihm. Go browse hobbypartz.com or hobbyking and you'll find cheap packs and a $30 or less charger.

I don't know I would need that small of a pack. I was prepared to fit a 1.3 x 1.7 x 2 inch pack. I'm not sure that they come in a more cube-ish shape rather than a flat pack. This is also mounted on the front axle. I have a servo mount made for it that provides 1 vertical wall for the servo and the battery is next to that. The only protection above and on the other side of the battery is an aluminum sheet metal guard I made. I'm not sure if that is good enough for something more dangerous like a Lipo.

Also, why would a smaller capacity pack last longer? Does a Lipo have more usable capacity before the voltage drops compared to a Nimh?

I also am looking for enough capacity for longer outings not just a few minutes on a comp course. I'm okay with carrying multiple packs, but not too many. 3 1500 mah Nimh packs served me well before as far as run time.

[redsawacs]

WORD!

Look Trash, we found something we both agree on - LOL! Kidding

[theTman]

Well, I looked at a lot of Lipos and I can't find any that would fit my crawler. The closest I could come to fitting would be 2s saddle pack, but even that is a little too large and would start hitting the shocks up front. This chassis was designed back when Lipo was pricey and less common place. It was built for an axle mounted battery so there is no room in the chassis.

The only other place that could fit it would be the back axle. I would have to fabricate a whole battery holding rig and I would also then have a lot of weight on the rear axle. I would also have to buy a new charger.

I think I would be better off sticking with Nimh for now. Eventually, once I get a new and better job and can afford it, I plan to get a SCX10 and then I might go Lipo. Just right now with my current rig it just isn't worth it, especially when it won't be seeing much use when I get a SCX10. I think 50$ for a couple 2100 Mah packs would serve me well until then. Honestly I can't even afford new batteries for this rig yet. Work has been giving me no hours whatsoever for a few weeks. I work at a sporting goods store and between the holidays and mid spring is a major slow time for business. I have to pinch every single penny right now. Once I get a new job I can afford to buy a couple packs since I can make enough to survive. The new rig will have to wait until I rebuild my cash cushion as it is pretty low.

[GySgt Hartman]

Originally Posted by djjiz

Run time doesn't change with more or less voltage.


And get a lipo.

Second fossil crawler thread, why are you talking to yourself jizz...?

[djjiz]

It's the only way to have a intellectual conversation.

[theTman]

yep, I'll admit it's a fossil. I just have the itch to get back into crawling and it's all I have at the moment.

This is the old dinosaur:

[theTman]

Well, I did some experimenting with my crawler and managed to pack a full sized flat pack into the chassis under the hood. I moved the ESC from under the front hood to being stuck upside down to the underside of the roof. The battery is under the hood above the links suspended in a battery sling I made out of a spare shoelace LOL! It holds the battery up and from moving side to side. I slide the battery in from the front and then tie it in with a second shoelace tied around the main sling.

The battery is nice and secure and the shoelaces are not in danger from the rocks. I'm not getting any Lipos any time soon as I can't afford a new charger.

[djjiz]

aka velcro

[6sharky9]

Plus one on using a lipo

[theTman]

Originally Posted by djjiz

aka velcro

I tried velcro straps first. I couldn't get a tight enough fit to keep the battery out of trouble.

[Olle P]

Originally Posted by djjiz

Run time doesn't change with more or less voltage.

Yes it does! (If the capacity is the same.)

The reason is that you don't actually pull current, but power, and higher voltage means more energy is stored.
6 cells á 1,500 mAh = 6*1.2*1.5 VAh = 10.8 VAh
8 cells á 1,500 mAh = 8*1.2*1.5 VAh = 14.4 VAh

Example:
You feed the motor 5.0V on average.
Using PWM regulation, as all of us do nowadays, that means with 7.5V battery voltage the power is "on" 2/3 of the time. With 10.0V battery voltage it's only on 1/2 of the time.

You increase the runtime by 1/3 of the original.

Now including the fact that we're talking NiMH packs, adding battery voltage while not increasing the (regulated) voltage drawn will really increase the useful runtime before the crawler begins to feel sluggish. (That's when the voltage wanted is more than the pack voltage.)