Lately they talk about the number of cells in a laptop battery, in the context of battery life. But that would mean the voltage varies, and I suspect that it's really the size of each cell that varies. Anyone know?
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Some Thinkpads come with optional six- or nine-cell batteries. The cells are exactly the same size, but the nine-cell batteries stick out the back of the computer.
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Obviously the battery must be bigger, but I asked if that was because the cells are bigger or because there are more cells.
As you say, in some cases there are more cells, so the voltage would be higher.
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In some cases I suspect they're parelleling strings -- I've used some HP laptops where you had your choice of 3-, 6-, or 9-cell packs, and I'd be surprised if they weren't just paralleling each set of 3 batteries in series so that they don't have to worry about large voltage changes.
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The ones I've seen are higher voltage. I'm not sure I've ever seen a notebook pack with parallel cells. Parallel cells would be ugly to charge.
The voltage change isn't a big deal. You build up a strings of batteries then buck convert to whatever you need. The switcher chips generally use an external fet for the high voltage path. The chip itself generally used a LDO to drop the voltage to something suitable for operation.
Laptop batteries these days are lithium-ion batteries. Such Li cells can be paralleled without problems. Most laptop batteries are 3 or 4 cells in series (nominal 10.8V or 14.4V), and any number of cells in parallel. The cells are first paralleled, then the parallel packs are connected in series, so that cell voltage in each cell can be controlled accurately to avoid over- or undercharge.
So, provided they use cells of the same capacity, a 9-cell battery would have 50% more capacity than a 6-cell.
Of course, using cells of higher capacity would give the same result as using a higher number of cells, only with a dramatic weight advantage. Such cells are, of course, more expensive, so manufacturers might want to boast about number of cells in stead of the battery's actual capacity, in order to save some money by using cheap cells.
When buying a laptop battery, look at its actual capacity, not number of cells.
-- RoRo
snipped-for-privacy@sushi.com Inscribed thus:
The vast majority use parallelled cells ! Most modern 3 cell packs use
3 X 3.4V cells in series giving about 10.2 volts.Also you will often find that each string has its own monitoring circuit. Including temperature, charge/discharge current and capacity.
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Best Regards:
Baron.
Hi Miso,
True, although there seem to be an awful lot of individual cell monitoring ICs out there these days... even from those folks like Maxim.
Plus I'm told that lithium ion, in particular, parallels quite nicely compared to NiCads/NiMHs -- that you can even ignore individual cell monitoring in some cases without much risk.
"The voltage change isn't a big deal. You build up a strings of batteries then buck convert to whatever you need."
Yeah, but if you're looking for the best efficiency it is harder to build a buck converter that operates over a 4:1 input range rather than the 1.25:1 that a single string gives you.
Still, I don't claim to actually know what I'm talking about here. :-)
Here's a list of a bunch of HP batteries and their rated voltages:
---Joel
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I did a quick survey of the protector chips. I can't find any that show batteries in parallel. Personally, I'd avoid such designs. I was looking at the Asus Eee PC 1005HA. It has 3 battery pack choices in two different voltages, so one could be parallel, or the cells could be bigger.
I rather engineer a wide range buck than deal with parallel cells, but hey, it's a free country.
Are you familiar with lithium battery technology? Li batteries have very different properties compared to Ni based batteries (NiCd and NiMH).
While Ni batteries have a very constant voltage regardless of charge state, Li batteries have a voltage that is strongly linked to charge state. When charging, full charge is reliably indicated by the cell voltage.
That means that two parallel cells will always have the same state of charge, as long as the cells are well matched.
It also means that connecting them in series *will* cause problems with imbalanced charge state, at least if the charge- and discharge currents are not small, and the battery is normally discharged deeply. That's why all charge controllers for high-performance Li batteries must monitor the voltage of each cell and adjust charge current for each in order to prevent the cells going out of balance.
In other words: Li batteries are the opposite of Ni based batteries: Well matched Li cells can be paralleled without issues, while serialling them requires extra precautions.
-- RoRo
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Cell quantity number may be somewhat a distracter. Each specific technology only supports a certain volumetric capacity. In the same volume and the exact same battery technology having more cells would actually decrease capacity. Similarly, small differences in technology can have interesting changes in capacity, cycle life and other properties.
Note that the cell balancing is done...history..fini...problem solved. Seems to me paralleling cells creates new problems to solve. Look at how many supposedly competent companies (re. Apple) screwed up lithium ion battery charging.
Screwing up charging Li-xx packs is fairly easy; the converse is that getting it right is somewhat harder. But it is not a black art at all, providing one inderstands the care and feeding requirements properly. The failures/recalls are *much* more to do with poor cells than the charging regimes.
As Robert stated, parallelling Li-xx cells is no drama, as they are a voltage-driven device and are therefore self-levelling in normal use. Ni-xx should be considered a current-driven device, which is why their serial strings tend to be "self-levelling" under overcharge (aka normal charge regime).
Series strings in Li-xx require monitoring, but rarely does a charger treat series elements with "adjusted charge". ("element" is a group of cells in parallel). Rather, the monitored imbalance is handled by terminating charge when the highest voltage element reaches the desired end-point (at which time the others are below full charge), and on discharge the shutoff occurs when the lowest SOC (voltage) element reaches the cutoff point (at which time the others are above the cutoff SOC). This creates an artificial reduction in usable capacity, which is most evident in devices such as laptops which largely use three and four-element strings (10.8/11.1 and 14.4/14.8 volts respectively). This can be verified by anyone who strips such a battery and checks the individual cells:
Cellphones - which can give cells a harder life than most laptops - avoid this issue by using a single element.
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None of what you describe makes me want to get a notebook with paralleled cells.
Think of the situation this way. The manufacturer picked a cell size they believe is maximum volume. If a double sized cell were considered appropriate, they would make such a cell. I have some old lithium ion manufacturers samples of very odd shapes. If there is a market, there is a way to serve the market.
Call me crazy, but I'll buy the high voltage pack given the choice over the "fat" pack.
At least now I have an idea how people are killing their packs in a year. Maybe the packs aren't all that rugged coming out the chute. The manufacturers are pressed for more "play time", parallel some cells, and the rest is history.
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