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Batteries + store tells me an alternator doesn't charge a battery...

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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Batteries + store tells me an alternator doesn't charge a battery...

Post by BlackBart » Mon May 06, 2019 3:21 pm

But isn't that what we grew up doing? Jump it, drive it around long enough to charge it, check it again the next morning, see if it's holding a charge...

They did volunteer that it's still under warranty and to drop it off and they will slow charge it for a day. So I will! It's got 12.4V, so about 75%..?


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by abscate » Mon May 06, 2019 3:24 pm

Probably someone trying to be helpful - if you are battery is dead, driving it home from work won't put enough charge into it to give it long life.

If you live in an area with a winter climate, you really should own a good battery charger. I loan mine out to neighbors, so I bought two.

I collect enough old batteries from people and bring them back to life with slow charging I think I have bought one battery for six cars in 25 years.


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by WhatAmIDoing » Mon May 06, 2019 3:36 pm

If your battery is DEAD, the alternator just isn't going to cut it after you jump it. The battery had a charge of 6.4V in the S70 I just bought. Once I pulled the jump pack off, car ran like crap because the alternator couldn't keep up with the load and charge the battery. Even after running for an hour, battery only showed 8.5 volts next day. Trickle charged it for 24hrs, battery shows 12.8 volts, starts right up every time since.

These cars don't like running off the alternator without a good battery to back them up.


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by BlackBart » Mon May 06, 2019 3:56 pm

Right. Well, we'll see what they say tomorrow.


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by jimmy57 » Mon May 06, 2019 6:10 pm

First, there is fully charged-acid at full strength and battery will live its full life charge. Then there is battery starts car and seems OK but the real charge level and acid strength is not at full capacity and battery will live a much shorter life than it is capable of charge.
Alternators until 6-7 years back had a simple charge plan: charge at higher voltage for 4-8 minutes and recover battery from starting engine and then voltage would go down to a level that supports the demands of vehicle with battery being on standby. If the battery was weak the short higher charge rate would only maybe get it to the starts car charge level but not to full capacity charged. Car is dead, jump it off and go drive it 10 minutes NEVER fully charged battery. The start/stop thing brought the need to REALLY charge the battery and measure its discharge and charge and to be capable of doing high charge until the battery is fully charged. Some 2011 Volvos (P3 platform) got the intelligent charge system and all current produced Volvos have it.



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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by tardcart » Mon May 06, 2019 6:30 pm

you wont get any warranty, a hot battery just off the charger will test fine. If there is a weak or dead cell it would have to sit over night or longer and be tested again and then it will fail. but they wont do tha, and you need a battery to drive. basically pep boys etc will never find a warranty problem under those conditions. If you really think its bad, you will have to dump all the acid out and replace with water to get a warranty claim.
as far as charging a dead battery slowly and rejuvenating it, ive only experienced that once. If a standard battery is dead for a week its done. also they dont load test them any more with a resistance checker. its some sort of gizmo that always says the battery is fine.



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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by DeadEric » Mon May 06, 2019 7:56 pm

Batteries + store is half right. The main job of a battery (besides starting) is to buffer the electrical system from the alternator.

Alternators generate a/c current and use a diode bridge to cut out the negative sine wave of the current. Even with the diode bridge you get a small amount of a/c current, called ripple, that the battery buffers from the system. The a/c ripple rides on top of d/c current and will cause very odd effects on d/c sensors.

The alternator will charge your battery if given enough time (several hours) but it only wants to maintain it.

Newer cars use a different strategy or smart charging. The idea is generating electricity puts demand on the engine, less demand, lower emissions. Even newer cars will have a clutch on the alternator (like an ac compressor) so the engine doesn't have to turn the generator when it's not needed. They use the reserve capacity of the battery to run the car and replenish the battery as needed and supplement the battery when current draw is high. Using this method they can use smaller alternators and have less emissions...

So now you have a dead/weak battery and the too small alternator is working double time to charge your monster of a battery (higher reserve capacity) and run the show. The computer takes notice and starts shutting down things that draw current but aren't vital to the car running or safety, known as load shedding.

The added heat and stress of working too hard burns up the alternator and now you get the sticker shock of an alternator with a clutch and a high reserve capacity battery.



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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by BlackBart » Fri May 10, 2019 5:46 pm

Conclusion!....

They kept the battery a couple of days, gave it an initial "boost" and then a trickle charge. Good as new they said, still under warranty.

I shouldn't have let it sit so long not knowing it was discharged.


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by wizechatmgr » Fri May 10, 2019 6:07 pm

Back in the day of having carbs some cars (<= 1975?) "could" charge the battery so to speak because they often had generators and not necessarily alternators - the only real loads they had were lights and radios. Now a days we're drawing far more due to the computer systems. Problem with most alternators is the field is provided by the existing battery - if it gets too low it almost collapses, causing very poor charging.

It wouldn't hurt to toss a solar tender on it if you have sunlight available. Make sure if you get one that it is regulated or you'll cook the battery.


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Re: Batteries + store tells me an alternator doesn't charge a battery...

Post by jimmy57 » Fri May 10, 2019 9:15 pm

Actually the field current is provided by the alternator through the magnetizing diodes once the alternator exceeds 500 or so engine RPM. The initial exciter supply for rotor is from battery but 8-9 V will be enough to give enough current for magnetic field needed to start output.

I disagree that an alternator on any Volvo model before the start/stop system came along will bring a battery back fully from a weak charge level. Once some sulphation is present the charge voltage has to be higher for longer to reverse sulphation to bring the battery back to its possible highest state of charge. Without the battery monitor sensor the detection of accurate charge state is not possible. With BMS there isn't a complete strategy to restore weak batteries until start stop arrived. If you low charge rate, 48 hour charge your battery twice per year you will make up for lack of the latest tech by bringing the battery back to its best possible state of charge. You need to do that 48 hr low charge if you ever have a greater discharge for any reason. The cars with BMS do a really job of taking care of their batteries and the needle is not moved that much by the newest tech. The newest tech does move the needle on a car with stop start compared to not having the best battery care strategy. Volvo uses the second battery on start stop system vehicles, I'm not sure how much that helps compared to makes using only the main battery.



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