All battery systems for electric vehicles, cars, boats etc should absolutely have a Battery Management System, a BMS. It doesn't matter if they are in series or parallel, every battery bank should have a BMS. This is a well-established practice with electric vehicle conversions. The general rule of thumb is that if you don't have a BMS, you're going to kill you battery pack before it normal life span.
It's also well established that when flooded lead acid batteries are used, 6 or 12v, that they should be all from the same pallet, which usually means that they were all manufactured on the same day, with the same physical make up of lead, of acid, same hands touching the same parts. Those minor differences over a period of time, with charging/discharging start adding up and after a certain point it gets very difficult to keep the pack balanced.
Rush
www.TucsonEV.com
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Friday, January 16, 2015 10:51 AM
To: electricboats@yahoogroups.com
Subject: Fwd: [Electric Boats] Charging batteries in series?
There are two phenomenon that batteries experience during charging - 1) slight differences in the resistance of the batteries and 2) slight differences in the amp hour capacity of the batteries.
When batteries are installed in series, each battery receives the same amount of current. Inconsistencies in the manufacturing processes, or aging can change the internal resistance of the different batteries.
A simple Ohms law calculation shows the increase in voltage that occurs as resistance changes. During the charge phase the weaker batteries show increased voltage due to the higher resistance. This is additive over time so every cycle, if left uncorrected, drives the batteries further out of balance.
The amp hour capacity decreases on the weakest battery so it becomes fully charged more quickly and before the other batteries. Once it reaches full charge, there are no more electrons available for transfer so all that is left is to electrolyse the water. This emits hydrogen gas. The hydrogen gas escapes and the battery starts losing its electrolyte.
Once the electrolyte vanishes, the battery fails.
If one battery becomes fully charged before all the rest of the batteries in the string, then the voltage of that battery can rise very quickly and steel the voltage away from all the other batteries in the string. It could very easy pop up to 18 or 20 volts while the other batteries are still at 12 volts. The weak battery gets over charged and the strong batteries don't get the full amount of charge which leads to the failure of every battery in the string.
The effect of these minor fluctuations becomes apparent over cycle life and time and drive the batteries out of balance with every cycle. It takes very small corrective action by putting energy into low cells (additive) or taking energy out of high cells (subtractive) to keep the batteries in balance.
During the charge cycle the weaker battery increases in voltage but during the discharge cycle the weaker battery decreases in voltage with respect to the other batteries in the string. We describe this as the weaker battery becoming spongy and not stiff like a good battery.
Effective Battery Balancing extends the life of your battery pack. Battery monitoring pays for itself over time plus gives the immediate satisfaction of knowing precisely the condition and capability of the energy system on board the boat.
Meters that measure voltage of the battery pack are not representative of the true energy remaining in the weakest battery, which determines the strength of the entire system.
In the coming months we are introducing an energy system for both Lead Acid and Lithium Batteries that balances batteries whenever a delta voltage or temperature event occurs.
It is advisable to monitor all batteries on a boat for voltage and temperature and make all decisions with charging based on individual battery conditions instead of pack voltage. Pack voltage is completely unrepresentative of what is happening at the individual battery level.
Complete integration of all loads and charging sources we call Integrated Hybrid Technology. This will become front row center in the quest to build safer boats. Having this technology aboard a boat makes your boat safer than any other diesel, gas or electric boat out there.
Safety will become the driving sales factor for any propulsion technology in the future, so you will hear more about this in the coming months and years as electric boats become more commonplace.
James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
On Jan 16, 2015, at 6:54 AM, John Acord jcacord@gmail.com [electricboats] wrote:
For years I have used flooded cells in series/parallel circuit and never noticed this. Having completed the motor installation in my boat I have just received my set of four 12V AGM 100 AH batteries and charger, Lifeline batteries and Qui-Q 48V charger with temperature compensation.
Off the truck the individual batteries terminal voltages varied by about 60 mV, not unexpected. I put each through a short discharge cycle and then wired up in series to charge them. At the start of the charging cycle I noticed quite a variation in terminal voltage of each battery which I expected. What I did not expect was that as the current tapered off, less than three amps, the differences in voltage between individual batteries increased quite noticeably. I would have expected that each battery would have accepted the charge it needed and the voltages would have become close to the same for each battery.
Would it have been better to charge each battery individually before putting them into series?
After putting the battery stack through a few cycles should the difference in battery voltages go away?
Any thoughts?
thanks,
John Acord
--Flatwater Electronics
www.flatwaterfarm.com
"Neurosurgery for computer looms."
Posted by: "Rushd Ironandwood" <Rushd@ironandwood.org>
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