Adding batteries for longer EOC

[MetroMPG]

Poor Daox. Ask a simple question....

[Trevs]

Some good info on here. There are some things that I have been wondering about that seem to have been covered. Thanks for the read.

[blueflame]

Why not sell the spare batts and buy led headlights?

My headlights get weak too on a glide, but I never checked the voltage, which must dip a lot.

My batt is old but bigger... on a 954cc car.

I live with it.

I keep some spare batts for my car collection but they are trickle charged every week.

[almightybmw]

Be careful with large capacity batteries. If you don't charge them fully, they will slowly die. And if you add those extra batteries, look at motorhomes for battery wiring. Coach and chassis batteries, both charged by the alternator, but when certain conditions arise (ie engine off) the chassis battery is disconnected from the coach battery, so not to drain it. That ensures you don't drain your battery so you can't restart the motor.

Make sense? I couldn't create a diagram for you, but it's a very simple system using diodes for isolation. Keeps the batteries from charging/discharging each other. The alternator will charge both, but when you EOC only the second battery will drain, your primary will stay healthy for the next morning when you need it.

[McTimson]

Out of curiosity, how long are your EOC glides? My voltage only drops to a little below 12V with lights on, but it's only about a 1.5 mile coast.

[Daox]

Originally Posted by MetroMPG

Poor Daox. Ask a simple question....

lol seriously, wow... I know there are better ways to do this guys. I just wanted an answer on the parallel battery question.


Thanks for the links Metro. Thats kind of what I figured. The larger capacity batteries will try to charge the smaller ones. I don't really see this as bad as I'm still increasing capacity and that is the goal.

Mctimson, length of coast isn't the problem. The engine off time is. Near the last leg of my trip I have a relatively short, but slow 25 mph zone I go through. Its slightly downhill and I probably keep my engine off for quite a while. I never timed it, but its between 30 seconds and a minute I'd think. On Wednesday I EOCed almost as much as I normally would with the headlights off. I think I cycled my battery down to 11.4V during that segment just to see how low it would get. Multiple times during the trip I was down into the mid 11V range. That is not good and will kill a battery pretty fast.

Anyway, last night I hooked up the two additional batteries in parallel. Guess what, it worked. My battery voltages were much more stable. As you'd expect, the voltage dropped slower. It also went up slower which is to be expected. The lowest voltage I saw on my trip in today was 11.9V. It did take a while for the batteries to build up a charge though. They all started out at 12.2V because they weren't all fully charged (my fault, I was rushed last night). So, I still needed to do engine on coasting to get their charge up initially, but after they were charged up a bit things went smoothly. The trip home tonight (no headlights) should give them their first good charge though.

[MetroMPG]

Glad it worked, Tim. 2 batteries in the back seat floor... hmm, looks familiar! I really need to get mine into a safer location and tied down when I go back to alternator-optional motoring this summer.

FYI - hit the jack pot:

"OK, but not optimal"

http://www.homepower.com/article/?file=HP122_pg14_ATE_5

Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity.

A bit more to be read at that link.

[MetroMPG]

Jackpot, part le deux:

Emergency Power from Solar ... - Google Book Search

The result of these [parallel battery] voltage differences can lead to inconsistencies in the treatment received by each battery (cell) in the bank, potentially causing unequal capacities within the bank. The parallel strings with the lowerst circuit resistance to the charging source will generally be exercised to a greater extent than the parallel groups of batteries with the greater circuit resistance to the charging source. The batteries in paralle strings which receive less charge may begin to sulfate prematurely

AND

Practical considerations : BATTERIES AND POWER SYSTEMS

we must be sure that these batteries are of equal voltage. If not, we will have relatively large currents circulating from one battery through another, the higher-voltage batteries overpowering the lower-voltage batteries.

I guess it's all about forming the right Google search terms...

[bwilson4web]

Well done!

My drill instructor used to say,"A grain of observation out weights a pound of <bolvine fecal matter> any day."

Let us know how the experiment comes out.

Bob Wilson

[NiHaoMike]

Have you thought about making a charger with a wall wart power supply?