Canister Light Battery Upgrade

[ronrosa]

My big question has always been, at what discharge rate can you run with those NiMHs? I cringe a bit when I see the SLA's running even 50W bulbs.

I bought a 12v 4ah pack based on 10 "C" cells for $25 as a test. Worked great with (1) 50 watt halogen bulb. At 45 minutes the battery was still holding fine and the lamp color was good.

I tried (1) 100 watt bulb and the battery's internal safety thermostat tripped. 100 watts at 12volts is about 8 amps, or double the rating of the battery pack. I recall reading somewhere about NIMH discharge rate recommendations of 1.5 the battery ah.

 

[in_cavediver]

I don't have actual experience with charging parallel packs, but the company I bought my pack and charger from doesn't recommend it. Here is a quote from their website.

"Why is putting batteries in parallel a bad thing?
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Discharging NiMH or NiCD in parallel is not a problem. Charging however is. When Ni based batteries are being charged their voltage rises until they are nearly full, then the voltage drops. 'Smart' chargers take advantage of this to detect when the batteries are full. It's called negative delta V detection. If two strings of cells are in parallel while being charged and one string fills first its voltage will drop and 'hog' all of the charge current. This will result in overcharging/overheating. "

Halycon's larger NIMH packs are parallel and I know a few diver's have homemade packs in parallel.

I guess if the charger has a thermister that would prevent overheating/overcharging of the "hot" pack, but what about the other pack ? Will the parallel pack be fully charged ? I don't know.

Lucky for me 10 "D" cells in series provides enough amps and burn time for my needs.

I would guess the problem is not with the batteries but with the charger itself. Simply put, two packs in parallel will see the same charger voltage and get the appropriate current based on impediance of the pack, which should be very similar since they are hard wired together and of similar age/abuse. The main issue is the charger and the current it can supply versus voltage. It is quite possible that an undersized charger is being used.

Mike

 

[in_cavediver]

My big question has always been, at what discharge rate can you run with those NiMHs? I cringe a bit when I see the SLA's running even 50W bulbs.

These are published with the manufacturer's Spec's. For instance, for the good ole 12V 7A/hr SLA, the max current listed for standard terminals is 40A. Mind you, a peak of 210A is supported. You do want to avoid full cycling the SLA's as they do have a limited life based on cycle depth. You also want to avoid drastically overdriving them as you can build up heat and all kinds of other nasties, which normally aren't much of a problem but can be when sealed in a little canister. The discharge rate though isn't the big issue. Time would be before depleting the battery.

NiMh are a differnet animal wih differing rules. The best bet is to consult the manufacturers documentation and then read about what happens to the capability curve a you increase the current (discharge rate). Most if not all NiMh packs should have a thermistor to protect the battery's for thermal abuse. Then simply choose accordingly.

Mike

 

[DeepScuba]

Hey Incave diver, I think we're talking about 2 different things, because I can tell you there's no way a 12V 7Ah battery can support a 40amp draw without dying 2 minutes afterwards.

If a 100W bulb is only drawing roughly 8.3 amps, it would be like saying you could use 1 battery to power almost 5 100W bulbs!!

Well I guess you could, but it would do it only once

I like ronrosa's rule of thumb. 1.5 times the Ah rating. Sounds reasonable.

What I'm referring to is a "reasonable" current draw. For SLA's (12V/7Ah) the ballpark is about 3 to 5 amps. Come to think of it, the NiMH I would probably use the same numbers..............

So I come full circle and ask myself, why WOULDN'T I just use cheap SLA's then?

 

[ronrosa]

So I come full circle and ask myself, why WOULDN'T I just use cheap SLA's then?

Smaller and lighter battery pack/canister, plus double the burn time ?

 

[in_cavediver]

Hey Incave diver, I think we're talking about 2 different things, because I can tell you there's no way a 12V 7Ah battery can support a 40amp draw without dying 2 minutes afterwards.

Well, yes and no. Most 500 VA computer UPS systems use the same ole 12V/7Ah battery (I have installed a lot of APC's like this). With that, you figure 500VA at 120V is about 4A. 500VA at 12V is about 40A, neglecting losses in conversion and inversion. So realistically, when the power goes off and the battery supplies juice, you'll see a load of 20-30A to support the PC. It may run 3-8 minutes before the low voltage cutoff kicks in but it still runs.

I am not advocating designing a dive light around this, I merely stating that SLA's can tolerate very high current draw's. There main limiting factor where you want to operate on the duty cycle range of the battery. Even at that, with a 75%+ capacity usage, you still get 250 cycles before the battery give out.

If a 100W bulb is only drawing roughly 8.3 amps, it would be like saying you could use 1 battery to power almost 5 100W bulbs!!

Well I guess you could, but it would do it only once

I like ronrosa's rule of thumb. 1.5 times the Ah rating. Sounds reasonable.

What I'm referring to is a "reasonable" current draw. For SLA's (12V/7Ah) the ballpark is about 3 to 5 amps. Come to think of it, the NiMH I would probably use the same numbers..............

So I come full circle and ask myself, why WOULDN'T I just use cheap SLA's then?

I would agree on the reasonable part. What you are really trying to avoid is popping a one way vents on your battery. High discharge rates and high charge rates can generate a lot of heat in the cell. Heat equals pressure and this pressure would pop the valve. (ruining the usefulness of the battery)

For the discharge rate, I would look to keep it more like 1.0x the Ah rating. 1.5x may be a bit high. 12.5A would generate quite a bit of heat and load (150 Watt bulb). I would also recommend sizing the battery for a 50% duty cycle on charging for your normal needs. IE, I need a light for 1 hour and the bulb draws 3.6A, choose 7Ah battery.

For my dual 75 watt video light, (almost done!), I am using dual 12V/7Ah batteries which puts me within the Ah rating. (12.5 A)

And lastly. I personally would look for NiMh batteries only for an HID light. Halogens tend to deep cycle batteries more due to the greatly increased current. That fact makes it a lot more appealing to use cheaper SLA's that are easily charged and replaced. (deeper cycle=lower number of total cycles)

Mike

 

[unfix8r]

I thought I was smart when I went to the computer recycler and bought a used computer backup. ($5) It's computer controlled and is designed exclusively for the "standby" 12v 7Ah battery. Thus far it's fully charged the battery each time and I can leave it in there when I'm not diving... It checks the battery output every 4 hours to insure this. If it needs juice, it gives it.

What do you guys think of this solution? Am I using the right charger or kidding myself that I'm so "smart"?

 

[DeepScuba]

Ron:

No it's not, (burn time) on a halogen. That's my point.

Read incave's post above. He summed it up perfectly!

NiMH's are a waste of money when driving a halogen bulb, so I'm finding out.

I think the best "rating" for an SLA for divelight use, is what's called the RC (Reserve Capacity). It's sometimes put on RV or Boat deep-cycle batteries. It's the time, in minutes, that a battery can be discharged at XX Amps and still have a voltage of 10.5V

I may be a tad off here, as I'm doing it from memory, but it's close.
Once the voltage drops below this, it's getting fairly low and starts to turn yellow

 

[DeepScuba]

unfix8r:

If it is designed for SLA's then it would work, but honestly, the thing wasn't design to "charge" batteries for a living, but it's more of a battery "maintainer".

How long will it work? Who knows, but it only cost $5.00

Battery chargers are real cheap too. And that's their full-time job.

 

[ronrosa]

Ron:

No it's not, (burn time) on a halogen. That's my point.

Read incave's post above. He summed it up perfectly!

NiMH's are a waste of money when driving a halogen bulb, so I'm finding out.

I guess I'm going to find out for myself since I already ordered a custom 9ah NIMH pack. I plan on only using a single 100 watt halogen lamp, so hopefully that won't be too hard on the pack. The $25 4ah NIMH pack seemed to do fine on repeated burn tests with a single 50 watt halogen lamp.