Alkaline vs. Rechargeable Batteries

sjenny

I am considering a switch from a 4-battery alkaline pack to a 5-battery rechargeable pack for powering the Boe-Bots for my classes of high school students. I notice that the brownout level is reached when my 5-battery rechargeable pack drops to about 6.3 v but the Boe-Bot is still running when the 4-battery alkaline voltage has dropped to 5.8 v. Sorry if this is an elementary question, but why is the brownout level different for rechargeables and alkalines?

Thank you,
Susan Jenny

Ken Gracey

Hello sjenny and welcome to the forums!

We'll receive some qualified replies on this issue aside from mine, as I'm only speculating because I've never seen this exact question before but found it interesting. I'm wondering if the rechargeable batteries are unable to source the current at 6.3V, as if they're on the steeper side of their discharge curve. If this is the case, maybe you could experience a brownout reset with a higher voltage.

By the way, have you ever seen the Boe-Bot Lithium Ion Power Pack? At $50 it's sorta expensive but it'll have many other benefits to your class. The storage capacity of these two 18650 Li-on batteries is significant, so you'll be able to get through many more sessions without recharging. http://www.parallax.com/product/28988

Let's wait for some others to chime in.

Ken Gracey

Mike Green

The brownout level is not any different with the rechargables. As Ken mentioned, the rechargables may drop their voltage more rapidly under load when nearly exhausted than the alkalines.

Genetix

What brands and types of batteries are you using?

Also, are you using a BOE-Boost (30078) with the rechargeables?

Phil Pilgrim (PhiPi)

Susan,

Please be careful with battery chargers! I would recommend having two sets of batteries and charge them only when you're in the classroom. IOW, don't leave batteries to charge overnight. And don't scrimp when it comes to purchasing a battery charger. I say this because I used a cheap charger from RadioShack once that melted, smoked, and nearly caught fire while charging some NiMH AA's. Had I not been there to catch it, the collateral damage could have been extensive.

I currently use an Ansmann charger, but I've heard that chargers from Maha are also good.

-Phil

lanternfish

I have recently experienced quality issues with Alkaline batteries. I chose a particular brand based on local (NZ) entertainment industry feedback and we have all noticed recently that batches of the particular brand can vary greatly in terms of discharge curves. Even batteries from the same batch can exhibit variance.

In my case it was powering headset mic belt packs. Even though each pack would have it' batteries replaced before each performance som batteries were failing during the performance.

On the upside, the supplier was more than happy to replace/refund any batteries that had underperformed and for the season this was some 20+ batteries out of approx. 300.

erco

I have been running my Boebots for some time now using two cheap (Gordon would say "non-prime") Li-ion cells from Ebay. I have tried the 18650s and the nearly AA-sized 14500 cells, and they're all about 800 mAH capacity. That said, they (and their chargers) are cheap and work well enough. Obviously higher voltage, and less weight so the bot is faster and more responsive. 8V might be pushing it on the servos, but I haven't smoked one yet. For a classroom environment, I'd recommend the larger 18650 cells for 2 reasons. First, they are far easier to remove from their oversized battery holder for charging (14500s are a bit longer than AAs and are a tight fit in an AA battery holder). Second and more importantly, it's only a matter of time before someone assumes the 14500 is an ordinary AA and shoves four of them into something else expecting 6V and gets 16V instead. Magic smoke!

http://www.ebay.com/itm/AC-US-Plug-Wired-Dual-Charger-for-18650-3-7v-Li-ion-Rechargeable-Battery-Cell-/310739121220?pt=Battery_Chargers&hash=item48597eb444#ht_4477wt_1366
http://www.ebay.com/itm/Black-Plastic-Battery-Storage-Case-Box-Holder-for-2-x-18650-with-6-Wire-Leads-/300836544851?pt=LH_DefaultDomain_0&hash=item460b416153
http://www.ebay.com/itm/UltraFire-18650-3-7v-3000mAh-Protected-Rechargeable-Battery-for-Flashlight-HE-/141081173579?pt=US_Rechargeable_Batteries&hash=item20d917de4b#ht_2334wt_1366

US seller. 4 cells: http://www.ebay.com/itm/4x-Ultrafire-18650-3-7v-4000mah-Rechargeable-Flashlight-Li-ion-Battery-fast-ship-/151176971883?pt=US_Rechargeable_Batteries&hash=item2332d9866b#ht_1082wt_1108

Don't believe any claims of 3000-4000 mAH. Expect 800-1000 mAH.

sjenny

Many thanks for the great replies. I am planning to purchase 5-battery holders rather than BOE-boosts because the 5-battery holders are a bit less expensive ($2.99 each for a 5-battery holder vs. $4.99 each for a 1-battery BOE-boost). Since budget is a consideration, I'm trying to use the NiMH Energizer and Duracell rechargeable batteries I have on-hand.The Energizer cells are 2300mAh and the Duracell cells are 2650 mAh. I am planning to purchase additional chargers and wondered if anyone has personal experience with Tenergy 10-cell or 12-cell chargers?

Thanks again.

LoopyByteloose

I certainly understand that you want to make sure that you have a cost effective solution for your school program.

Everyone has pretty much covered the salient points.
A. The brown out point is the same, regardless of what you think your are getting as different results
B. The rechargible batteries tend to have much flatter curves with steeper tails at each end.

I would go with the 5 cell holder as you will recapture your expenses from the change-over more quickly than buying lithium cell systems. Plus, you can use a variety of rechargeable cells. You can buy NiCd or NiMH or whatever is on sale at any given time. You have a lot more leeway in economizing on your future battery purchases.

Why are you getting different readings for these?
It might be your procedure, or it might be that the rechargable cells 'bounce back' quickly from a deep discharge. Just about any battery will 'bounce back' from a low discharge.. but they all do so at different rates. The alkaline might just not bounce as quickly or as much.

The Lithium cells are a much nicer system. Parallax has a holder with built in charger, so cells do not have to be removed. You don't have to buy a separate charger, and the Lithium cells do indeed recharge must faster. You can possibly recharge between lessons that are scheduled for the same day. The NiCd and NiMH tend to require an overnight charge. You end up with buying more cells and having boxes of extras. Some one constantly has to sort out which are good, which need recharge, and which are defective trash.

User Name

I have no experience with Tenergy chargers but thought I'd mention that the La Crosse Technology BC-700 charger is another great choice of smart charger that won't overcharge. It's less expensive than the Maha. I have both, and like both, but use the La Crosse more often. Amazon often has a good deal going.

Mark_T

I would also advise checking the quality of the battery holders - cheap ones are not even soldered and rely on
mechanical contact between a spring and a rivet - the result is a much larger contact resistance than the
internal resistance of most batteries.

I guess this means check that the voltage at the battery matches that at the holder terminals when under
load.

I've been caught out by this before - I will not trust battery bolders unless soldered or welded contacts
are used throughout.

ratronic

Phil Pilgrim (PhiPi) wrote: »

I've heard that chargers from Maha are also good.

+1 on Maha. I use a Maha MH-C801D 8 cell charger and charge Tenergy 2600mah 1.2V AA batteries for my S2 robot NiMh batteries